diff --git a/modules/sensors/README.md b/modules/sensors/README.md
index 132a91ec..0cf08be2 100644
--- a/modules/sensors/README.md
+++ b/modules/sensors/README.md
@@ -28,13 +28,29 @@ seat's verbs. The full reasoning is in ADR 0256.
 |---|---|---|
 | `smartmontools` | the package: `smartctl` and `smartd` | |
 | `/etc/smartd.conf` | every drive (`DEVICESCAN`), every check, never waking a drive in standby, no mail | written whole |
-| `smartd.service` drop-in | runs on a virtual machine too, and stays up with no drive to watch (`-q never`) | written whole |
+| `smartd.service` drop-in | runs on a virtual machine too, stays up with no drive to watch (`-q never`), and checks each drive hourly (`-i 3600`) | written whole |
 | `smartd` | watches every drive between looks, and logs what it finds to the journal | root, the package's unit |
-| `watch` (`sensors watch`) | every 10 min each drive's health with `smartctl --json -a -n standby`; every 30 s which fans turn; one file, `/run/mesh-sensors/readings.json` (0644) | root, the node-engine's process |
-| the tools | read `/sys`, `/proc` and the watch's file | the operator account, in the tool runner |
+| `watch` (`sensors watch`) | each drive's health with `/usr/bin/smartctl --json -a -n standby,2,3`: a spinning drive hourly, any other every 10 min; every 30 s which fans turn; one file, `/run/mesh-sensors/readings.json` (0644, in a 0755 directory) | root, the node-engine's process |
+| the tools | read `/sys`, `/proc` and the watch's file, once per call | the operator account, in the tool runner |
 
-The tools need no root, because the watch has already read what only root may read. A drive in standby
-is never woken. The watch keeps its last reading of that drive and says that it was asleep.
+The tools need no root, because the watch has already read what only root may read.
+
+**The drives never hold up the fans.** The watch reads the drives beside its looks at the fans, one pass
+at a time. Each smartctl is bounded by 20 s, plus 2 s for a child that keeps its output open. The file is
+written every 30 s whatever a drive does, so a file older than five minutes means the watch is hung or
+gone, and the health check says so.
+
+**A drive is never woken, and is read seldom enough to sleep.**
+- smartctl's `-n standby,2,3` stops with status 2 and the drive's power mode when the drive is in standby
+  or asleep. The watch then keeps the reading before and says the drive was asleep. A drive is asleep only
+  on status 2 with a parsed power mode (or smartctl's own "STANDBY (OS)").
+- It stops with status 3 when the drive cannot say (a USB bridge that does not pass CHECK POWER MODE on).
+  Such a drive is not read, so that reading it never wakes it, and it is said as not read.
+- Reading SMART from an awake drive can restart its standby timer, so a drive read more often than its
+  timer is set for never spins down. No mesh tool reads a drive's standby timer, so whether the servers'
+  drives spin down is not known. A spinning drive is therefore read hourly, by the watch and by smartd.
+  A drive whose timer is set longer than an hour may still be kept awake.
+- smartd has no per-drive equivalent of status 3. A drive behind such a bridge may be woken by smartd.
 
 ## Tools
 
@@ -55,20 +71,26 @@ All six only read.
 |---|---|
 | temperature HIGH | at or above its own maximum, or a zone's `hot` trip (without one, its lowest `passive` trip) |
 | temperature CRITICAL | at or above its own critical or emergency value, or a zone's `critical` trip |
-| drive FAILING | SMART's overall verdict failed, an attribute is failing now, or an NVMe critical warning bit is set |
-| drive HIGH | reallocated, pending or unreadable sectors; an attribute that failed in the past; NVMe media errors or 90 % of rated endurance used; errors in the error log; a last self-test that did not pass |
-| fan STOPPED | its chip's alarm, or under its own minimum, once it has been seen turning; or it ran since boot and reads 0 while a temperature is at or above its own maximum |
+| drive FAILING | SMART's overall verdict failed (SATA), a pre-failure attribute is failing now, or an NVMe critical warning bit other than temperature is set |
+| drive HIGH | an old-age attribute failing now; an attribute that failed in the past; the NVMe temperature bit alone; reallocated, pending or unreadable sectors; NVMe media errors or 90 % of rated endurance used; errors in the error log; a last self-test that did not pass |
+| fan STOPPED | it ran since boot, reads 0 now, and the CPU, the board or a sensor on its own chip is at or above its own maximum |
+| fan note | under its chip's minimum, or its chip's alarm bit set: said, never flagged |
 | memory HIGH / CRITICAL | corrected / uncorrectable errors since boot |
 | PCIe HIGH | fatal or non-fatal errors since boot |
 
-**What is no threshold.** A threshold given as the format's largest value is no threshold. The laptop's
-NVMe drive gives 65261.85 °C as the maximum of two of its sensors. A threshold of 0 is no threshold
-either, and neither is anything outside −40 to 200 °C. The reading is then said without a flag. A CPU
-sensor that gives no threshold, such as AMD's `Tctl`, is never flagged.
+**What is no threshold, and no reading.**
+- A threshold given as the format's largest value is no threshold. The laptop's NVMe drive gives
+  65261.85 °C as the maximum of two of its sensors.
+- A threshold of 0 is no threshold either, and neither is anything outside −40 to 200 °C. The reading
+  is then said without a flag.
+- A CPU sensor that gives no threshold, such as AMD's `Tctl`, is never flagged.
+- 0 °C is a reading. 127 and 127.5 °C are not: they are what a board chip's input with nothing on it
+  answers (a phantom AUXTIN). Such a sensor is said as not read.
 
-**Why a fan at 0 is not enough.** Laptop fans and GPU fans in zero-fan mode stop on purpose when the
-machine is cool. A board header with nothing on it reads 0, and may carry an alarm or a minimum that the
-firmware set. So a fan is judged only once it has been seen turning.
+**Why a fan at 0, a minimum or an alarm is not enough.** Laptop fans and GPU fans in zero-fan mode stop on
+purpose when the machine is cool, under the minimum their chip gives. A board chip latches an alarm bit,
+and a header with nothing on it reads 0. Only a fan that ran and stopped while what it cools is hot is a
+fault. A hot drive or network adapter says nothing of a fan.
 
 ## Health
 
@@ -80,15 +102,18 @@ The module declares its health under ADR 0240:
 
 `sensors_health` is unhealthy when:
 
-- a temperature is at or above its own critical value;
-- a drive says it is failing;
-- a fan that ran has stopped while the machine is hot;
-- memory reports an uncorrectable error.
+- a CPU, GPU, drive or zone temperature is at or above its own critical value. A board chip's (Super I/O)
+  critical value is shown CRITICAL by the tools and never makes the module unhealthy: its limits are
+  whatever the firmware left;
+- a drive says it is failing (the FAILING row above);
+- a fan is STOPPED (the row above);
+- memory reports an uncorrectable error;
+- the watch has written no readings since the machine started, or none for five minutes.
 
 Its `why` is one plain sentence per finding, for the operator. For example: "The drive sdb (WDC
-WUH722222ALE6L4) says it is failing: its overall SMART health check failed." HIGH readings raise nothing.
-A drive whose health was not read is said in `why` and does not make the check unhealthy. The watch not
-running is caught by the watch's own liveness.
+WUH722222ALE6L4) says it is failing: its overall SMART health check failed." HIGH readings and fan notes
+raise nothing. A drive whose health was not read (asleep since boot, or behind a bridge) is said in `why`
+and does not make the check unhealthy.
 
 The condition this raises is `module.sensors.<machine>.unhealthy`. ADR 0256 records what this costs:
 
@@ -100,7 +125,7 @@ The machine judging its own hardware, as it judges its network, is the follow-up
 ## Tests
 
 `go test ./...`. The tests read a tree standing in for `/sys`, `/proc` and `/run`, and smartctl answers
-recorded as smartmontools 7 writes them. There are three machines:
+as smartmontools 7.5 writes them (its standby answer as its source sets it). There are three machines:
 
 - a laptop, as the laptop showed itself on 2026-10-08;
 - a server with ECC memory, three SATA drives read by drivetemp and a board chip with an empty fan header;
@@ -110,14 +135,20 @@ They cover:
 
 - thresholds taken from the hardware only, and a zone's hwmon copy left out;
 - HIGH at the maximum and CRITICAL at the critical value; a zone's passive trip as its high;
-- a sensor that gives no reading said as not read;
+- 0 °C as a reading, a phantom 127 °C as not read, and a board chip's critical value raising nothing;
 - every class not present on the virtual machine, which is healthy;
-- a failing SATA drive, an NVMe critical warning, and early signs that are HIGH and raise nothing;
-- a missing, stale or earlier-boot reading said as such;
-- a fan at rest, a fan that stopped while hot, an empty header, and a fan under its minimum;
+- a failing SATA drive; an NVMe critical warning; the NVMe temperature bit alone as HIGH; NVMe without
+  `smart_status`; an old-age attribute failing now and a past failure as HIGH;
+- a missing, stale or earlier-boot reading, and the watch's staleness making the module unhealthy;
+- a fan at rest, a fan that stopped while the board is hot, a hot drive saying nothing of a fan, a hot
+  sensor on the fan's own chip, an empty header, a zero-fan GPU under its minimum, and a latched alarm;
 - memory and PCIe errors, and CPU throttling;
-- the watch reading as root without waking a drive, keeping a reading for a drive asleep, reading drives
-  only every 10 min, and keeping what it saw across a restart;
+- the watch reading as root without waking a drive: smartctl's real standby answer, an open failure
+  that is not sleep, and a bridge without a power mode;
+- a spinning drive read hourly and a solid-state one every 10 min;
+- a smartctl that hangs with a child holding its output, bounded, and the fans looked at and written
+  while a drive hangs;
+- a restart keeping what the last watch read; the readings directory given its mode;
 - a watch that is not root asking through `sudo -n`, and saying what stops it;
 - the health answer in the shape the node-engine reads;
 - the manifest: the tools listed are the tools served, all read-only, no seat claimed, no machine named,
diff --git a/modules/sensors/cmd/sensors/drives.go b/modules/sensors/cmd/sensors/drives.go
index 4fc01a85..ed4de019 100644
--- a/modules/sensors/cmd/sensors/drives.go
+++ b/modules/sensors/cmd/sensors/drives.go
@@ -18,9 +18,13 @@ import (
 // ReadingsPath is the watch's one file: what it last read of each drive, and which fans it saw turning.
 const ReadingsPath = "/run/mesh-sensors/readings.json"
 
-// StaleAfter is how old the watch's file may be before the tools say the watch may not be running: three
-// of its drive readings missed.
-const StaleAfter = 30 * time.Minute
+// StaleAfter is how old the watch's file may be before the module is unhealthy: the watch writes it every
+// FanEvery, so ten looks missed is a watch that is hung or gone, never a slow drive (drives are read
+// beside the fans, not before them).
+const StaleAfter = 5 * time.Minute
+
+// Smartctl is the program, by its path: the watch runs as root and runs nothing found on a PATH.
+const Smartctl = "/usr/bin/smartctl"
 
 // FlagFailing is said beside a drive that says it is failing.
 const FlagFailing = "FAILING"
@@ -43,6 +47,8 @@ type DriveReading struct {
 	// one before.
 	Asleep  bool   `json:"asleep,omitempty"`
 	NotRead string `json:"not_read,omitempty"`
+	// LookedAt is the newest look at the drive, asleep or not: what the next look is timed from.
+	LookedAt time.Time `json:"looked_at"`
 }
 
 // Drive is one drive, as the tools say it.
@@ -144,10 +150,10 @@ func (m *Machine) ReadReadings() (*Readings, string) {
 	return &r, ""
 }
 
-// Drives is every drive with its health as the watch last read it, judged.
-func (m *Machine) Drives() ([]Drive, string) {
+// Drives is every drive with its health as the watch's readings say it, judged. r is nil, with why, when
+// there are none to use.
+func (m *Machine) Drives(r *Readings, why string) ([]Drive, string) {
 	drives := m.BlockDrives()
-	r, why := m.ReadReadings()
 	note := why
 	if r != nil && m.Now().Sub(r.WrittenAt) > StaleAfter {
 		note = fmt.Sprintf("the module's watch last wrote %s ago; it may not be running", ago(m.Now().Sub(r.WrittenAt)))
@@ -163,7 +169,7 @@ func (m *Machine) Drives() ([]Drive, string) {
 			d.Health, d.NotRead = "not read", "the module's watch has not read this drive yet"
 			continue
 		}
-		judgeDrive(d, reading, m.Now())
+		judgeDrive(d, reading)
 	}
 	return drives, note
 }
@@ -196,6 +202,9 @@ type smartJSON struct {
 	SmartStatus *struct {
 		Passed bool `json:"passed"`
 	} `json:"smart_status"`
+	PowerMode *struct {
+		Name string `json:"name"`
+	} `json:"power_mode"`
 	NVMe *struct {
 		CriticalWarning         int64 `json:"critical_warning"`
 		AvailableSpare          int64 `json:"available_spare"`
@@ -211,7 +220,10 @@ type smartJSON struct {
 			Value      int64  `json:"value"`
 			Thresh     int64  `json:"thresh"`
 			WhenFailed string `json:"when_failed"`
-			Raw        struct {
+			Flags      struct {
+				Prefailure bool `json:"prefailure"`
+			} `json:"flags"`
+			Raw struct {
 				Value int64 `json:"value"`
 			} `json:"raw"`
 		} `json:"table"`
@@ -259,7 +271,7 @@ func num(v int64) *int64 { return &v }
 
 // judgeDrive reads one drive's smartctl answer into the drive, with its findings: FAILING when the drive
 // says so itself, HIGH for the early signs a drive gives before it fails.
-func judgeDrive(d *Drive, reading DriveReading, now time.Time) {
+func judgeDrive(d *Drive, reading DriveReading) {
 	if !reading.ReadAt.IsZero() {
 		d.ReadAt = reading.ReadAt.UTC().Format(time.RFC3339)
 	}
@@ -276,7 +288,7 @@ func judgeDrive(d *Drive, reading DriveReading, now time.Time) {
 		d.Model = s.ModelName
 	}
 	d.Serial, d.Firmware = s.SerialNumber, s.FirmwareVersion
-	if s.SmartStatus == nil {
+	if s.SmartStatus == nil && s.NVMe == nil {
 		d.Health = "not read"
 		switch {
 		case s.SmartSupport != nil && !s.SmartSupport.Available:
@@ -290,7 +302,9 @@ func judgeDrive(d *Drive, reading DriveReading, now time.Time) {
 	}
 	d.Health = "passed"
 	var failing, high []string
-	if !s.SmartStatus.Passed {
+	// An NVMe drive's overall verdict is its critical warning byte, so it is judged from the bits below;
+	// the temperature bit alone is a drive running hot, not one failing.
+	if s.SmartStatus != nil && !s.SmartStatus.Passed && s.NVMe == nil {
 		failing = append(failing, "its overall SMART health check failed")
 	}
 	if s.Temperature != nil && s.Temperature.Current != 0 {
@@ -306,7 +320,11 @@ func judgeDrive(d *Drive, reading DriveReading, now time.Time) {
 			d.PowerOnHours = num(n.PowerOnHours)
 		}
 		for _, w := range nvmeWarnings {
-			if n.CriticalWarning&w.bit != 0 {
+			switch {
+			case n.CriticalWarning&w.bit == 0:
+			case w.bit == 0x02:
+				high = append(high, w.said)
+			default:
 				failing = append(failing, w.said)
 			}
 		}
@@ -329,7 +347,14 @@ func judgeDrive(d *Drive, reading DriveReading, now time.Time) {
 			}
 			switch at.WhenFailed {
 			case "now":
-				failing = append(failing, fmt.Sprintf("its attribute %s is at %d, at or below its own threshold of %d", at.Name, at.Value, at.Thresh))
+				// A pre-failure attribute at its threshold is the drive saying it is about to fail; an
+				// old-age one (a temperature, a count of hours) is wear or a hot day.
+				said := fmt.Sprintf("its attribute %s is at %d, at or below its own threshold of %d", at.Name, at.Value, at.Thresh)
+				if at.Flags.Prefailure {
+					failing = append(failing, said)
+				} else {
+					high = append(high, said+" (an old-age attribute)")
+				}
 			case "past":
 				high = append(high, fmt.Sprintf("its attribute %s was at or below its own threshold in the past", at.Name))
 			}
@@ -380,18 +405,21 @@ func firstError(s smartJSON, fallback string) string {
 }
 
 // ReadDrive asks smartctl for one drive's health, as root (directly when this process is root, else
-// through sudo -n), without waking a drive in standby. A drive asleep keeps the reading before.
+// through sudo -n), without waking a drive in standby. `-n standby,2,3` makes smartctl stop with status 2
+// and the drive's power mode when it is asleep, and with status 3 when the drive cannot say (a USB bridge
+// that does not pass the question on): such a drive is not read, because reading it could wake it. A
+// drive asleep keeps the reading before.
 func (m *Machine) ReadDrive(device string, before *DriveReading) DriveReading {
-	args := []string{"--json=c", "-a", "-n", "standby", "/dev/" + device}
-	program := "smartctl"
+	args := []string{"--json=c", "-a", "-n", "standby,2,3", "/dev/" + device}
+	program := Smartctl
 	if m.UID != 0 {
-		program, args = "sudo", append([]string{"-n", "smartctl"}, args...)
+		program, args = "sudo", append([]string{"-n", Smartctl}, args...)
 	}
 	r := m.Run(context.Background(), program, args...)
 	now := m.Now()
-	out := DriveReading{ReadAt: now, Status: r.Status}
+	out := DriveReading{ReadAt: now, LookedAt: now, Status: r.Status}
 	switch {
-	case r.Err == "ENOENT" && program == "smartctl":
+	case r.Err == "ENOENT" && program == Smartctl:
 		out.NotRead = "smartctl is not installed; the module declares smartmontools, so the package has not been applied yet"
 		return out
 	case r.Err == "ENOENT":
@@ -407,17 +435,21 @@ func (m *Machine) ReadDrive(device string, before *DriveReading) DriveReading {
 		out.NotRead = "the watch is not root and may not run smartctl through sudo without a prompt"
 		return out
 	}
-	said := strings.ToUpper(r.Stdout + r.Stderr)
-	if r.Status&2 != 0 && (strings.Contains(said, "STANDBY") || strings.Contains(said, "SLEEP")) {
+	var s smartJSON
+	parsed := json.Unmarshal([]byte(r.Stdout), &s) == nil
+	switch {
+	case parsed && r.Status == 2 && asleep(s):
 		if before != nil {
 			kept := *before
-			kept.Asleep = true
+			kept.Asleep, kept.LookedAt = true, now
 			return kept
 		}
 		out.Asleep, out.NotRead = true, "the drive was asleep at every look, and is not woken to be read"
 		return out
-	}
-	if !json.Valid([]byte(r.Stdout)) {
+	case parsed && r.Status == 3 && saysNoPowerMode(s):
+		out.NotRead = "the drive cannot say whether it is asleep (a USB bridge, usually), so it is not read, never to wake it"
+		return out
+	case !parsed:
 		out.NotRead = "smartctl answered something other than JSON (status " + strconv.Itoa(r.Status) + ")"
 		return out
 	}
@@ -425,6 +457,32 @@ func (m *Machine) ReadDrive(device string, before *DriveReading) DriveReading {
 	return out
 }
 
+// asleep says smartctl stopped because the drive is in standby or asleep: by the power mode it parsed,
+// or by its own word that the operating system has the drive powered down.
+func asleep(s smartJSON) bool {
+	if s.PowerMode != nil {
+		switch s.PowerMode.Name {
+		case "STANDBY", "STANDBY_Y", "SLEEP":
+			return true
+		}
+	}
+	for _, msg := range s.Smartctl.Messages {
+		if strings.HasPrefix(msg.String, "Device is in STANDBY (OS) mode") {
+			return true
+		}
+	}
+	return false
+}
+
+func saysNoPowerMode(s smartJSON) bool {
+	for _, msg := range s.Smartctl.Messages {
+		if strings.HasPrefix(msg.String, "CHECK POWER MODE not implemented") || strings.HasPrefix(msg.String, "CHECK POWER MODE returned unknown value") {
+			return true
+		}
+	}
+	return false
+}
+
 // WriteReadings writes the watch's file whole, through a rename, readable by every account: it holds no
 // secret, and the tools read it as the operator account.
 func (m *Machine) WriteReadings(r Readings) error {
@@ -432,6 +490,10 @@ func (m *Machine) WriteReadings(r Readings) error {
 	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return err
 	}
+	// MkdirAll leaves a directory that was already there as it was: the tools must be able to enter it.
+	if err := os.Chmod(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
 	raw, err := json.Marshal(r)
 	if err != nil {
 		return err
diff --git a/modules/sensors/cmd/sensors/fans.go b/modules/sensors/cmd/sensors/fans.go
index d65344f5..be63d415 100644
--- a/modules/sensors/cmd/sensors/fans.go
+++ b/modules/sensors/cmd/sensors/fans.go
@@ -2,13 +2,16 @@ package main
 
 import (
 	"fmt"
+	"path"
 	"strings"
 )
 
 // Fans, from the hwmon chips. Many fans stop on purpose when the machine is cool (a laptop at rest, a GPU
-// in its zero-fan mode), so 0 RPM alone is not a fault. A fan is said STOPPED only when the hardware says
-// so itself (its alarm bit, or a reading under its own minimum) or when it ran since the machine started
-// and reads 0 while a temperature is at or above the hardware's own maximum (novox/hq ADR 0256 rule 4).
+// in its zero-fan mode), so 0 RPM alone is not a fault; nor is a chip's minimum or alarm bit, which a
+// zero-fan GPU is under by design and a Super I/O chip latches. Those are said in a note and raise nothing.
+// A fan is STOPPED only when it ran since the machine started, reads 0 now, and a temperature that fan
+// cools is at or above the hardware's own maximum: the CPU's, the board's, or one on the fan's own chip
+// (novox/hq ADR 0256 rule 4).
 
 // FlagStopped is said beside a fan that should be turning and is not.
 const FlagStopped = "STOPPED"
@@ -28,8 +31,10 @@ type Fan struct {
 	RanSinceBoot *bool  `json:"ran_since_boot,omitempty"`
 	Flag         string `json:"flag,omitempty"`
 	Why          string `json:"why,omitempty"`
-	NotRead      string `json:"not_read,omitempty"`
-	Where        string `json:"where"`
+	// Note is what the chip says that raises nothing: under its own minimum, its alarm bit.
+	Note    string `json:"note,omitempty"`
+	NotRead string `json:"not_read,omitempty"`
+	Where   string `json:"where"`
 }
 
 // Name is how a person reads which fan this is.
@@ -70,10 +75,9 @@ func (m *Machine) Fans() []Fan {
 	return out
 }
 
-// judgeFans flags each fan by what the hardware says of it and, given which fans ran since the machine
-// started (nil when the watch has not said) and the hottest temperature at or above its own maximum, a fan
-// that stopped while the machine is hot.
-func judgeFans(fans []Fan, ran map[string]bool, hot *Temperature) []Fan {
+// judgeFans flags each fan, given which fans the watch saw turning since the machine started (nil when it
+// has not said) and the temperatures read now.
+func judgeFans(fans []Fan, ran map[string]bool, temps []Temperature) []Fan {
 	for i := range fans {
 		f := &fans[i]
 		if ran != nil {
@@ -83,31 +87,40 @@ func judgeFans(fans []Fan, ran map[string]bool, hot *Temperature) []Fan {
 		if f.RPM == nil {
 			continue
 		}
-		// A header with nothing on it reads 0 and may carry an alarm or a minimum the firmware set: a fan
-		// is judged only once it has been seen turning, now or since the machine started.
-		ranBefore := f.RanSinceBoot != nil && *f.RanSinceBoot
-		seen := *f.RPM > 0 || ranBefore
-		switch {
-		case f.Alarm && seen:
-			f.Flag, f.Why = FlagStopped, fmt.Sprintf("its chip raises its alarm at %d RPM", *f.RPM)
-		case f.Min != nil && *f.RPM < *f.Min && seen:
-			f.Flag, f.Why = FlagStopped, fmt.Sprintf("it turns at %d RPM, under its own minimum of %d RPM", *f.RPM, *f.Min)
-		case *f.RPM == 0 && ranBefore && hot != nil:
-			f.Flag, f.Why = FlagStopped, fmt.Sprintf("it ran earlier and reads 0 RPM while %s is at %.0f °C, at or above its own maximum of %.0f °C",
-				hot.Name(), *hot.C, *hot.High)
+		var notes []string
+		if f.Alarm {
+			notes = append(notes, fmt.Sprintf("its chip's alarm bit is set at %d RPM", *f.RPM))
+		}
+		if f.Min != nil && *f.RPM < *f.Min {
+			notes = append(notes, fmt.Sprintf("it turns at %d RPM, under its chip's minimum of %d RPM", *f.RPM, *f.Min))
+		}
+		f.Note = strings.Join(notes, "; ")
+		if *f.RPM != 0 || f.RanSinceBoot == nil || !*f.RanSinceBoot {
+			continue
+		}
+		if hot := hottestFor(*f, temps); hot != nil {
+			f.Flag, f.Why = FlagStopped, fmt.Sprintf("it ran earlier and reads 0 RPM while %s is at %.1f °C, at or above %s of %.1f °C",
+				hot.Name(), *hot.C, hot.HighIs, *hot.High)
 		}
 	}
 	return fans
 }
 
-// hottest is the temperature furthest at or above its own maximum, or nil when none is.
-func hottest(ts []Temperature) *Temperature {
+// hottestFor is the temperature furthest at or above its own high value among those this fan may be
+// cooling: the CPU's, the board's, or one on the fan's own chip. A drive or a network adapter running hot
+// says nothing of a fan.
+func hottestFor(f Fan, ts []Temperature) *Temperature {
+	chip := path.Dir(f.Where)
 	var best *Temperature
 	for i := range ts {
 		t := &ts[i]
 		if t.C == nil || t.High == nil || *t.C < *t.High {
 			continue
 		}
+		related := t.Kind == "cpu" || t.Kind == "board" || (t.Source == "hwmon" && path.Dir(t.Where) == chip)
+		if !related {
+			continue
+		}
 		if best == nil || *t.C-*t.High > *best.C-*best.High {
 			best = t
 		}
diff --git a/modules/sensors/cmd/sensors/findings.go b/modules/sensors/cmd/sensors/findings.go
index 37d646b3..6c68e524 100644
--- a/modules/sensors/cmd/sensors/findings.go
+++ b/modules/sensors/cmd/sensors/findings.go
@@ -15,21 +15,32 @@ type Look struct {
 	Drives       []Drive
 	DrivesNote   string
 	Faults       Faults
+	// Watch is why the watch's readings are not to be trusted now: none, from an earlier boot, or older
+	// than StaleAfter. Empty when they are fresh.
+	Watch string
 }
 
-// LookNow reads every class, and judges the fans against what the watch saw turning since the machine
-// started.
+// LookNow reads every class, the watch's file once, and judges the fans against what the watch saw
+// turning since the machine started.
 func (m *Machine) LookNow() Look {
 	l := Look{Temperatures: m.Temperatures(), Fans: m.Fans(), Faults: m.Faults()}
-	l.Drives, l.DrivesNote = m.Drives()
+	r, why := m.ReadReadings()
+	l.Drives, l.DrivesNote = m.Drives(r, why)
 	var ran map[string]bool
-	if r, why := m.ReadReadings(); why == "" && r != nil {
+	switch {
+	case r == nil:
+		l.Watch = "The module's watch has left no readings since the machine started, so drive health and fan history are not being read."
+	case m.Now().Sub(r.WrittenAt) > StaleAfter:
+		l.Watch = fmt.Sprintf("The module's watch has not written its readings for %s, so drive health and fan history are not being read.",
+			ago(m.Now().Sub(r.WrittenAt)))
+	}
+	if r != nil {
 		ran = map[string]bool{}
 		for k := range r.FansRan {
 			ran[k] = true
 		}
 	}
-	l.Fans = judgeFans(l.Fans, ran, hottest(l.Temperatures))
+	l.Fans = judgeFans(l.Fans, ran, l.Temperatures)
 	return l
 }
 
@@ -37,8 +48,8 @@ func (m *Machine) LookNow() Look {
 func (l Look) Problems() []string {
 	var out []string
 	for _, t := range l.Temperatures {
-		if t.Flag == FlagCritical {
-			out = append(out, fmt.Sprintf("%s is at %.0f °C, at or above %s of %.0f °C.", capital(t.Name()), *t.C, t.CriticalIs, *t.Critical))
+		if t.Flag == FlagCritical && t.DrivesHealth() {
+			out = append(out, fmt.Sprintf("%s is at %.1f °C, at or above %s of %.1f °C.", capital(t.Name()), *t.C, t.CriticalIs, *t.Critical))
 		}
 	}
 	for _, d := range l.Drives {
@@ -57,6 +68,9 @@ func (l Look) Problems() []string {
 				c.Uncorrectable, c.Controller))
 		}
 	}
+	if l.Watch != "" {
+		out = append(out, l.Watch)
+	}
 	return out
 }
 
@@ -79,7 +93,7 @@ func (l Look) Health() Health {
 				unread++
 			}
 		}
-		// Said, never passed as read. A watch that is not running is the watch's liveness to judge.
+		// Said, never passed as read: a drive asleep since boot, or one the watch cannot read.
 		if unread > 0 {
 			why += fmt.Sprintf(" The health of %d of %d drives has not been read.", unread, len(l.Drives))
 		}
@@ -128,6 +142,9 @@ func (l Look) Said() []string {
 			out = append(out, fmt.Sprintf("HIGH: the PCIe device %s (%s) counted %d fatal and %d non-fatal errors since the machine started", e.Device, orUnknown(e.Driver), e.Fatal, e.NonFatal))
 		}
 	}
+	if l.Watch != "" {
+		out = append(out, "STALE: "+l.Watch)
+	}
 	return append(out, l.overview()...)
 }
 
diff --git a/modules/sensors/cmd/sensors/helpers_test.go b/modules/sensors/cmd/sensors/helpers_test.go
index c43945d9..b3ceb255 100644
--- a/modules/sensors/cmd/sensors/helpers_test.go
+++ b/modules/sensors/cmd/sensors/helpers_test.go
@@ -181,9 +181,32 @@ const (
 		`"ata_smart_error_log":{"summary":{"count":0}},"ata_smart_self_test_log":{"standard":{"table":[{"type":{"string":"Short offline"},"status":{"string":"Completed without error","passed":true}}]}},` +
 		`"temperature":{"current":36},"power_on_time":{"hours":17000}}`
 	ataFailing = `{"smartctl":{"exit_status":24},"model_name":"WDC  WUH722222ALE6L4","serial_number":"A2","smart_support":{"available":true,"enabled":true},"smart_status":{"passed":false},` +
-		`"ata_smart_attributes":{"table":[{"id":5,"name":"Reallocated_Sector_Ct","value":1,"thresh":10,"when_failed":"now","raw":{"value":4012}},` +
+		`"ata_smart_attributes":{"table":[{"id":5,"name":"Reallocated_Sector_Ct","value":1,"thresh":10,"when_failed":"now","flags":{"prefailure":true},"raw":{"value":4012}},` +
 		`{"id":197,"name":"Current_Pending_Sector","value":100,"thresh":0,"when_failed":"","raw":{"value":16}}]},"ata_smart_error_log":{"summary":{"count":12}}}`
 	ataPending = `{"smartctl":{"exit_status":64},"model_name":"WDC  WUH722222ALE6L4","serial_number":"A3","smart_support":{"available":true,"enabled":true},"smart_status":{"passed":true},` +
 		`"ata_smart_attributes":{"table":[{"id":197,"name":"Current_Pending_Sector","value":100,"thresh":0,"when_failed":"","raw":{"value":8}}]},"ata_smart_error_log":{"summary":{"count":0}}}`
 	noSMART = `{"smartctl":{"exit_status":4,"messages":[{"string":"Read Device Identity failed: Unknown","severity":"error"}]},"smart_support":{"available":false}}`
 )
+
+type ctxT = context.Context
+
+// smartctlLine is the command the watch runs for one drive, as root.
+func smartctlLine(device string) string {
+	return "/usr/bin/smartctl --json=c -a -n standby,2,3 /dev/" + device
+}
+
+func chmod(path string, mode os.FileMode) error { return os.Chmod(path, mode) }
+
+func modeOf(t *testing.T, path string) os.FileMode {
+	t.Helper()
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return info.Mode().Perm()
+}
+
+// fresh writes readings the watch has just written, with every drive of the server passed.
+func (f *fake) fresh() {
+	f.readings(f.now, bootID, map[string]string{"sda": ataPassed, "sdb": ataPassed, "sdc": ataPassed})
+}
diff --git a/modules/sensors/cmd/sensors/machine.go b/modules/sensors/cmd/sensors/machine.go
index 925b3669..9c63e7d8 100644
--- a/modules/sensors/cmd/sensors/machine.go
+++ b/modules/sensors/cmd/sensors/machine.go
@@ -22,8 +22,13 @@ import (
 // node-engine runs as root, reads it with smartctl and leaves its readings in one file the tools read
 // (novox/hq ADR 0256).
 
-// CommandTimeout bounds one smartctl: a drive that does not answer costs one line, never the watch.
-const CommandTimeout = 20 * time.Second
+// CommandTimeout bounds one smartctl: a drive that does not answer costs one line, never the watch. A
+// variable only so a test can shorten it.
+var CommandTimeout = 20 * time.Second
+
+// WaitDelay is how long a command ended for its timeout may keep its output open (a child it left behind)
+// before the pipes are closed under it: without it, the wait for the output outlives the timeout.
+const WaitDelay = 2 * time.Second
 
 // Ran is what one command did: its output and exit status, and why it never ran to an answer.
 type Ran struct {
@@ -43,12 +48,13 @@ func ExecRunner(ctx context.Context, name string, args ...string) Ran {
 	defer cancel()
 	cmd := exec.CommandContext(ctx, name, args...)
 	cmd.Env = append(os.Environ(), "LC_ALL=C")
+	cmd.WaitDelay = WaitDelay
 	var out, errb bytes.Buffer
 	cmd.Stdout, cmd.Stderr = &out, &errb
 	err := cmd.Run()
 	r := Ran{Stdout: out.String(), Stderr: errb.String()}
-	if ctx.Err() == context.DeadlineExceeded {
-		r.Status, r.Err = 124, fmt.Sprintf("no answer within %d s", int(CommandTimeout.Seconds()))
+	if ctx.Err() == context.DeadlineExceeded || errors.Is(err, exec.ErrWaitDelay) {
+		r.Status, r.Err = 124, fmt.Sprintf("no answer within %s", CommandTimeout)
 		return r
 	}
 	var exit *exec.ExitError
diff --git a/modules/sensors/cmd/sensors/manifest_test.go b/modules/sensors/cmd/sensors/manifest_test.go
index 9fcf9829..11b2d93d 100644
--- a/modules/sensors/cmd/sensors/manifest_test.go
+++ b/modules/sensors/cmd/sensors/manifest_test.go
@@ -79,7 +79,7 @@ func TestTheHealthIsTheDriveDaemonsUnitAndTheWatchsToolCheck(t *testing.T) {
 		t.Errorf("smartd never wakes a drive and sends no mail: %q", conf)
 	}
 	dropIn, _ := byID["smartd-drop-in"]["content"].(string)
-	for _, want := range []string{"ConditionVirtualization=\n", "ExecStart=\nExecStart=/usr/bin/smartd -n -q never\n"} {
+	for _, want := range []string{"ConditionVirtualization=\n", "ExecStart=\nExecStart=/usr/bin/smartd -n -q never -i 3600\n"} {
 		if !strings.Contains(dropIn, want) {
 			t.Errorf("the drop-in misses %q", want)
 		}
diff --git a/modules/sensors/cmd/sensors/review_test.go b/modules/sensors/cmd/sensors/review_test.go
new file mode 100644
index 00000000..f5460042
--- /dev/null
+++ b/modules/sensors/cmd/sensors/review_test.go
@@ -0,0 +1,294 @@
+package main
+
+// The reproductions of the review of 2026-10-08 (mesh-catalog #129): each fails on the first build of the
+// module and passes on the fix.
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+)
+
+const (
+	// smartmontools 7.5's own answer for a drive in standby with -n standby,2,3: the power mode is set
+	// before the information message, and the exit status is the one -n asked for (ataprint.cpp).
+	standbyJSON = `{"json_format_version":[1,0],"smartctl":{"version":[7,5],"messages":[{"string":"Device is in STANDBY mode, exit(2)",` +
+		`"severity":"information"}],"exit_status":2},"device":{"name":"/dev/sda","info_name":"/dev/sda [SAT]","type":"sat","protocol":"ATA"},` +
+		`"power_mode":{"ata_value":0,"name":"STANDBY"}}`
+	// A bridge that cannot answer CHECK POWER MODE, with -n standby,2,3: exit 3 and no power mode.
+	noPowerModeJSON = `{"smartctl":{"messages":[{"string":"CHECK POWER MODE not implemented, exit(3)","severity":"information"}],` +
+		`"exit_status":3},"device":{"name":"/dev/sdd","type":"sat","protocol":"ATA"}}`
+	// A drive that could not be opened, whose model happens to hold the word SLEEP: exit 2, no power mode.
+	openFailedJSON = `{"smartctl":{"messages":[{"string":"Smartctl open device: /dev/sda failed: No such device","severity":"error"}],` +
+		`"exit_status":2},"model_name":"SLEEPY DRIVE"}`
+	nvmeHot = `{"smartctl":{"exit_status":8},"model_name":"Samsung SSD 990 PRO 2TB","smart_status":{"passed":false,"nvme":{"value":2}},` +
+		`"nvme_smart_health_information_log":{"critical_warning":2,"available_spare":100,"available_spare_threshold":10,"percentage_used":1,"media_errors":0}}`
+	nvmeNoStatusDegraded = `{"smartctl":{"exit_status":0},"model_name":"Old NVMe","nvme_smart_health_information_log":{"critical_warning":4,"available_spare":100,` +
+		`"available_spare_threshold":10,"percentage_used":20,"media_errors":0}}`
+	nvmeNoStatusWell = `{"smartctl":{"exit_status":0},"model_name":"Old NVMe","nvme_smart_health_information_log":{"critical_warning":0,"available_spare":100,` +
+		`"available_spare_threshold":10,"percentage_used":20,"media_errors":0}}`
+	// An old-age attribute failing now, and another that failed in the past; the overall verdict passed.
+	ataOldAge = `{"smartctl":{"exit_status":32},"model_name":"ST16000NM001G","smart_support":{"available":true,"enabled":true},"smart_status":{"passed":true},` +
+		`"ata_smart_attributes":{"table":[{"id":5,"name":"Reallocated_Sector_Ct","value":100,"thresh":10,"when_failed":"","flags":{"prefailure":true},"raw":{"value":0}},` +
+		`{"id":194,"name":"Temperature_Celsius","value":40,"thresh":45,"when_failed":"now","flags":{"prefailure":false},"raw":{"value":60}},` +
+		`{"id":190,"name":"Airflow_Temperature_Cel","value":40,"thresh":45,"when_failed":"past","flags":{"prefailure":false},"raw":{"value":60}}]}}`
+)
+
+func TestTheNVMeTemperatureBitAloneIsHighNotFailing(t *testing.T) {
+	f := newFake(t)
+	f.laptop()
+	f.readings(f.now, bootID, map[string]string{"nvme0n1": nvmeHot})
+	l := f.machine().LookNow()
+	d := l.Drives[0]
+	if d.Flag != FlagHigh || d.Health != "passed" {
+		t.Errorf("the temperature bit alone is HIGH, and the verdict it implies is not a failure: %+v", d)
+	}
+	if !l.Health().Healthy {
+		t.Errorf("health = %+v", l.Health())
+	}
+}
+
+func TestNVMeWithoutSmartStatusIsJudgedFromItsHealthLog(t *testing.T) {
+	f := newFake(t)
+	f.laptop()
+	f.readings(f.now, bootID, map[string]string{"nvme0n1": nvmeNoStatusDegraded})
+	if d := f.machine().LookNow().Drives[0]; d.Flag != FlagFailing {
+		t.Errorf("a degraded log is failing without smart_status: %+v", d)
+	}
+	f.readings(f.now, bootID, map[string]string{"nvme0n1": nvmeNoStatusWell})
+	if d := f.machine().LookNow().Drives[0]; d.Health != "passed" || d.Flag != "" {
+		t.Errorf("a clean log has passed: %+v", d)
+	}
+}
+
+func TestAnOldAgeAttributeFailingNowAndAPastFailureAreHigh(t *testing.T) {
+	f := newFake(t)
+	f.server()
+	f.readings(f.now, bootID, map[string]string{"sda": ataOldAge})
+	l := f.machine().LookNow()
+	d := l.Drives[0]
+	if d.Flag != FlagHigh || d.Health != "passed" {
+		t.Fatalf("an old-age attribute and a past failure are HIGH: %+v", d)
+	}
+	joined := strings.Join(d.Findings, "; ")
+	for _, want := range []string{"Temperature_Celsius", "Airflow_Temperature_Cel was at or below its own threshold in the past"} {
+		if !strings.Contains(joined, want) {
+			t.Errorf("findings miss %q: %s", want, joined)
+		}
+	}
+	if !l.Health().Healthy {
+		t.Errorf("health = %+v", l.Health())
+	}
+}
+
+func TestAZeroFanGPUUnderItsMinimumIsNotStopped(t *testing.T) {
+	f := newFake(t)
+	f.laptop()
+	f.files("/sys/class/hwmon/hwmon12", map[string]string{"name": "amdgpu", "fan1_input": "0", "fan1_min": "500", "temp1_input": "45000", "temp1_label": "edge"})
+	f.readings(f.now, bootID, map[string]string{"nvme0n1": nvmePassed}, "amdgpu/fan1")
+	l := f.machine().LookNow()
+	var gpu Fan
+	for _, x := range l.Fans {
+		if x.Key == "amdgpu/fan1" {
+			gpu = x
+		}
+	}
+	if gpu.Flag != "" || gpu.Note == "" {
+		t.Errorf("a zero-fan GPU at rest is said, not flagged: %+v", gpu)
+	}
+	if !l.Health().Healthy {
+		t.Errorf("health = %+v", l.Health())
+	}
+}
+
+func TestALatchedAlarmOnATurningFanRaisesNothing(t *testing.T) {
+	f := newFake(t)
+	f.server()
+	f.file("/sys/class/hwmon/hwmon1/fan1_alarm", "1")
+	f.readings(f.now, bootID, map[string]string{"sda": ataPassed, "sdb": ataPassed, "sdc": ataPassed}, "nct6775/fan1")
+	l := f.machine().LookNow()
+	if l.Fans[0].Flag != "" || !strings.Contains(l.Fans[0].Note, "alarm") {
+		t.Errorf("a latched alarm while turning is said, not flagged: %+v", l.Fans[0])
+	}
+	if !l.Health().Healthy {
+		t.Errorf("health = %+v", l.Health())
+	}
+}
+
+func TestAnUnrelatedHotSensorDoesNotMakeAFanStopped(t *testing.T) {
+	f := newFake(t)
+	f.laptop()
+	f.file("/sys/class/hwmon/hwmon7/temp1_input", "84000") // the NVMe drive past its own maximum
+	f.readings(f.now, bootID, map[string]string{"nvme0n1": nvmePassed}, "asus/fan1", "asus/fan2")
+	l := f.machine().LookNow()
+	if l.Fans[2].Flag != "" {
+		t.Fatalf("a hot drive says nothing of the GPU fan: %+v", l.Fans[2])
+	}
+	// The board's zone at its hot trip does.
+	f.file("/sys/class/thermal/thermal_zone0/temp", "124000")
+	l = f.machine().LookNow()
+	if l.Fans[2].Flag != FlagStopped {
+		t.Fatalf("the board at its hot trip with a fan that ran reading 0: %+v", l.Fans[2])
+	}
+}
+
+func TestAFanIsStoppedByAHotSensorOnItsOwnChip(t *testing.T) {
+	f := newFake(t)
+	f.server()
+	f.files("/sys/class/hwmon/hwmon9", map[string]string{"name": "pwmchip", "fan1_input": "0", "temp1_input": "71000", "temp1_max": "70000"})
+	f.readings(f.now, bootID, map[string]string{"sda": ataPassed, "sdb": ataPassed, "sdc": ataPassed}, "pwmchip/fan1")
+	h := f.machine().LookNow().Health()
+	if h.Healthy || !strings.Contains(h.Why, "at 71.0 °C, at or above its own maximum of 70.0 °C") {
+		t.Errorf("health = %+v", h)
+	}
+}
+
+func TestAPhantom127SensorIsNotReadAndABoardChipsCriticalRaisesNothing(t *testing.T) {
+	f := newFake(t)
+	f.server()
+	f.files("/sys/class/hwmon/hwmon1", map[string]string{"temp1_input": "105000", "temp1_label": "SYSTIN", "temp1_crit": "100000",
+		"temp2_input": "127000", "temp2_label": "AUXTIN0", "temp2_crit": "100000", "temp3_input": "127500", "temp3_label": "AUXTIN1"})
+	f.readings(f.now, bootID, map[string]string{"sda": ataPassed, "sdb": ataPassed, "sdc": ataPassed})
+	l := f.machine().LookNow()
+	for _, x := range l.Temperatures {
+		if x.Chip != "nct6775" {
+			continue
+		}
+		switch x.Label {
+		case "SYSTIN":
+			if x.Flag != FlagCritical {
+				t.Errorf("SYSTIN is shown CRITICAL: %+v", x)
+			}
+		default:
+			if x.C != nil || x.NotRead == "" {
+				t.Errorf("%s at 127 °C is a phantom: %+v", x.Label, x)
+			}
+		}
+	}
+	if !l.Health().Healthy {
+		t.Errorf("a board chip's critical value drives no health: %+v", l.Health())
+	}
+}
+
+func TestZeroDegreesIsAReading(t *testing.T) {
+	f := newFake(t)
+	f.files("/sys/class/hwmon/hwmon0", map[string]string{"name": "outdoor", "temp1_input": "0"})
+	ts := f.machine().Temperatures()
+	if len(ts) != 1 || ts[0].C == nil || *ts[0].C != 0 {
+		t.Errorf("0 °C is a reading: %+v", ts)
+	}
+}
+
+func TestAMissingOrStaleWatchMakesTheModuleUnhealthy(t *testing.T) {
+	f := newFake(t)
+	f.laptop()
+	if h := f.machine().LookNow().Health(); h.Healthy || !strings.Contains(h.Why, "watch") {
+		t.Errorf("no readings at all: %+v", h)
+	}
+	f.readings(f.now.Add(-StaleAfter-time.Minute), bootID, map[string]string{"nvme0n1": nvmePassed})
+	if h := f.machine().LookNow().Health(); h.Healthy || !strings.Contains(h.Why, "has not written its readings for") {
+		t.Errorf("stale readings: %+v", h)
+	}
+	f.readings(f.now.Add(-time.Minute), bootID, map[string]string{"nvme0n1": nvmePassed})
+	if h := f.machine().LookNow().Health(); !h.Healthy {
+		t.Errorf("fresh readings: %+v", h)
+	}
+}
+
+func TestSmartctlsStandbyAnswerIsAsleepOnlyWithItsPowerMode(t *testing.T) {
+	f := newFake(t)
+	f.drive("sda", "X", "ATA     ", 100, true)
+	f.answers[smartctlLine("sda")] = Ran{Stdout: standbyJSON, Status: 2}
+	before := DriveReading{ReadAt: f.now.Add(-time.Hour), Smartctl: []byte(ataPassed)}
+	got := f.machine().ReadDrive("sda", &before)
+	if !got.Asleep || string(got.Smartctl) != ataPassed {
+		t.Errorf("standby keeps the reading before: %+v", got)
+	}
+	f.answers[smartctlLine("sda")] = Ran{Stdout: openFailedJSON, Status: 2}
+	got = f.machine().ReadDrive("sda", &before)
+	if got.Asleep {
+		t.Errorf("an open failure is not sleep, whatever its words: %+v", got)
+	}
+	f.answers[smartctlLine("sda")] = Ran{Stdout: noPowerModeJSON, Status: 3}
+	got = f.machine().ReadDrive("sda", nil)
+	if got.Asleep || !strings.Contains(got.NotRead, "cannot say whether it is asleep") {
+		t.Errorf("a bridge without a power mode is skipped and said: %+v", got)
+	}
+}
+
+func TestASmartctlThatHangsCostsOneLineAndTheFansGoOn(t *testing.T) {
+	old := CommandTimeout
+	CommandTimeout = 300 * time.Millisecond
+	defer func() { CommandTimeout = old }()
+	start := time.Now()
+	// A child that keeps the output open after its parent is ended.
+	r := ExecRunner(context.Background(), "sh", "-c", "sleep 30 & sleep 30")
+	if took := time.Since(start); took > 5*time.Second {
+		t.Fatalf("a hung command held the watch for %s", took)
+	}
+	if r.Err == "" {
+		t.Errorf("the hang is said: %+v", r)
+	}
+
+	f := newFake(t)
+	f.server()
+	release := make(chan struct{})
+	m := f.machine()
+	m.Run = func(_ ctxT, name string, args ...string) Ran { <-release; return Ran{Stdout: ataPassed} }
+	w := NewWatch(m)
+	w.Async = true
+	if err := w.Look(); err != nil {
+		t.Fatal(err)
+	}
+	f.now = f.now.Add(FanEvery)
+	if err := w.Look(); err != nil {
+		t.Fatal(err)
+	}
+	r2, why := m.ReadReadings()
+	if why != "" || !r2.WrittenAt.Equal(f.now) || len(r2.FansRan) != 1 {
+		t.Errorf("the fans are looked at and written while a drive hangs: %+v %q", r2, why)
+	}
+	close(release)
+	w.WaitDrives()
+}
+
+func TestASpinningDriveIsReadAtMostHourlyAndASolidStateOneEveryTenMinutes(t *testing.T) {
+	f := newFake(t)
+	f.server()
+	f.drive("nvme0n1", "N", "", 100, false)
+	for _, d := range []string{"sda", "sdb", "sdc"} {
+		f.answers[smartctlLine(d)] = Ran{Stdout: ataPassed}
+	}
+	f.answers[smartctlLine("nvme0n1")] = Ran{Stdout: nvmePassed}
+	w := NewWatch(f.machine())
+	if err := w.Look(); err != nil || len(f.calls) != 4 {
+		t.Fatalf("first look reads every drive: %q %v", f.calls, err)
+	}
+	f.calls = nil
+	f.now = f.now.Add(SolidStateEvery)
+	_ = w.Look()
+	if len(f.calls) != 1 || !strings.HasSuffix(f.calls[0], "/dev/nvme0n1") {
+		t.Fatalf("after ten minutes only the solid-state drive: %q", f.calls)
+	}
+	f.calls = nil
+	f.now = f.now.Add(SpinningEvery)
+	_ = w.Look()
+	if len(f.calls) != 4 {
+		t.Fatalf("after an hour every drive: %q", f.calls)
+	}
+}
+
+func TestAnExistingReadingsDirectoryGetsItsMode(t *testing.T) {
+	f := newFake(t)
+	f.dir("/run/mesh-sensors")
+	if err := chmod(f.root+"/run/mesh-sensors", 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := f.machine().WriteReadings(Readings{}); err != nil {
+		t.Fatal(err)
+	}
+	if mode := modeOf(t, f.root+"/run/mesh-sensors"); mode != 0o755 {
+		t.Errorf("mode %o", mode)
+	}
+}
diff --git a/modules/sensors/cmd/sensors/sensors_test.go b/modules/sensors/cmd/sensors/sensors_test.go
index 931a4b41..387ac156 100644
--- a/modules/sensors/cmd/sensors/sensors_test.go
+++ b/modules/sensors/cmd/sensors/sensors_test.go
@@ -54,6 +54,7 @@ func TestATemperatureIsHighAtItsMaximumAndCriticalAtItsCriticalValue(t *testing.
 	f.server()
 	f.file("/sys/class/hwmon/hwmon0/temp2_input", "80000")
 	f.file("/sys/class/hwmon/hwmon0/temp1_input", "100000")
+	f.fresh()
 	ts := f.machine().Temperatures()
 	if core := find(ts, "coretemp", "Core 0"); core.Flag != FlagHigh {
 		t.Errorf("at its own maximum it is HIGH: %+v", core)
@@ -63,7 +64,7 @@ func TestATemperatureIsHighAtItsMaximumAndCriticalAtItsCriticalValue(t *testing.
 		t.Fatalf("at its own critical value it is CRITICAL: %+v", pkg)
 	}
 	h := f.machine().LookNow().Health()
-	want := "The CPU (coretemp Package id 0) is at 100 °C, at or above its own critical value of 100 °C."
+	want := "The CPU (coretemp Package id 0) is at 100.0 °C, at or above its own critical value of 100.0 °C."
 	if h.Healthy || h.Why != want {
 		t.Errorf("health = %+v, want unhealthy with %q", h, want)
 	}
@@ -121,7 +122,7 @@ func TestADriveThatSaysItIsFailingMakesTheModuleUnhealthy(t *testing.T) {
 	f := newFake(t)
 	f.server()
 	f.readings(f.now, bootID, map[string]string{"sda": ataPassed, "sdb": ataFailing, "sdc": ataPending})
-	ds, note := f.machine().Drives()
+	ds, note := f.machine().Drives(f.machine().ReadReadings())
 	if note != "" || len(ds) != 3 {
 		t.Fatalf("three drives, the RAID volume left out, no note: %d, %q", len(ds), note)
 	}
@@ -146,7 +147,7 @@ func TestAnNVMeCriticalWarningBitIsFailing(t *testing.T) {
 	f := newFake(t)
 	f.laptop()
 	f.readings(f.now, bootID, map[string]string{"nvme0n1": nvmeSpareLow})
-	ds, _ := f.machine().Drives()
+	ds, _ := f.machine().Drives(f.machine().ReadReadings())
 	d := ds[0]
 	if d.Flag != FlagFailing || *d.CriticalWarning != 1 || *d.PercentageUsed != 97 || d.Model != "Samsung SSD 990 PRO 2TB" {
 		t.Fatalf("spare below threshold is failing: %+v", d)
@@ -183,11 +184,11 @@ func TestAHealthyLaptopReadsHealthyWithItsDriveRead(t *testing.T) {
 func TestAMissingStaleOrEarlierBootReadingIsSaid(t *testing.T) {
 	f := newFake(t)
 	f.laptop()
-	if ds, note := f.machine().Drives(); ds[0].Health != "not read" || !strings.Contains(note, "has left no reading") {
+	if ds, note := f.machine().Drives(f.machine().ReadReadings()); ds[0].Health != "not read" || !strings.Contains(note, "has left no reading") {
 		t.Errorf("no file: %+v %q", ds[0], note)
 	}
 	f.readings(f.now.Add(-2*time.Hour), bootID, map[string]string{"nvme0n1": nvmePassed})
-	if ds, note := f.machine().Drives(); ds[0].Health != "passed" || note != "the module's watch last wrote 2.0 h ago; it may not be running" {
+	if ds, note := f.machine().Drives(f.machine().ReadReadings()); ds[0].Health != "passed" || note != "the module's watch last wrote 2.0 h ago; it may not be running" {
 		t.Errorf("stale: %+v %q", ds[0], note)
 	}
 	f.readings(f.now, "an-earlier-boot", map[string]string{"nvme0n1": nvmeSpareLow}, "asus/fan2")
@@ -208,24 +209,25 @@ func TestAFanAtRestOnACoolMachineIsNotStopped(t *testing.T) {
 	}
 }
 
-func TestAFanThatRanAndReadsZeroWhileTheMachineIsHotHasStopped(t *testing.T) {
+func TestAFanThatRanAndReadsZeroWhileTheBoardIsHotHasStopped(t *testing.T) {
 	f := newFake(t)
 	f.laptop()
-	f.file("/sys/class/hwmon/hwmon7/temp1_input", "84000")
+	f.file("/sys/class/thermal/thermal_zone0/temp", "124500")
 	f.readings(f.now, bootID, map[string]string{"nvme0n1": nvmePassed}, "asus/fan1", "asus/fan2")
 	l := f.machine().LookNow()
 	if l.Fans[2].Flag != FlagStopped {
-		t.Fatalf("gpu_fan ran and reads 0 while the drive is past its maximum: %+v", l.Fans[2])
+		t.Fatalf("gpu_fan ran and reads 0 while the board is past its hot trip: %+v", l.Fans[2])
 	}
 	h := l.Health()
-	want := "The fan gpu_fan (asus) has stopped: it ran earlier and reads 0 RPM while the drive nvme0n1 (WD PC SN560 SDDPNQE-1T00-1102), " +
-		"sensor Composite is at 84 °C, at or above its own maximum of 84 °C."
+	want := "The board (acpitz thermal_zone0) is at 124.5 °C, at or above the zone's critical trip of 124.0 °C. " +
+		"The fan gpu_fan (asus) has stopped: it ran earlier and reads 0 RPM while the board (acpitz thermal_zone0) " +
+		"is at 124.5 °C, at or above the zone's hot trip of 124.0 °C."
 	if h.Healthy || h.Why != want {
 		t.Errorf("health = %+v\nwant %q", h, want)
 	}
 }
 
-func TestAnEmptyHeaderIsNotAStoppedFanAndAFanUnderItsMinimumIs(t *testing.T) {
+func TestAnEmptyHeaderAndAFanUnderItsMinimumAreSaidAndNotFlagged(t *testing.T) {
 	f := newFake(t)
 	f.server()
 	l := f.machine().LookNow()
@@ -234,14 +236,15 @@ func TestAnEmptyHeaderIsNotAStoppedFanAndAFanUnderItsMinimumIs(t *testing.T) {
 	}
 	f.file("/sys/class/hwmon/hwmon1/fan1_input", "200")
 	l = f.machine().LookNow()
-	if l.Fans[0].Flag != FlagStopped || l.Fans[0].Why != "it turns at 200 RPM, under its own minimum of 300 RPM" {
-		t.Errorf("under its own minimum: %+v", l.Fans[0])
+	if l.Fans[0].Flag != "" || l.Fans[0].Note != "it turns at 200 RPM, under its chip's minimum of 300 RPM" {
+		t.Errorf("under its own minimum is a note: %+v", l.Fans[0])
 	}
 }
 
 func TestMemoryErrorsAreCountedAndAnUncorrectableOneIsUnhealthy(t *testing.T) {
 	f := newFake(t)
 	f.server()
+	f.fresh()
 	f.file("/sys/devices/system/edac/mc/mc0/ce_count", "3")
 	f.file("/sys/devices/system/edac/mc/mc0/dimm0/dimm_ce_count", "3")
 	l := f.machine().LookNow()
@@ -277,9 +280,9 @@ func TestPCIeErrorsAndThrottlingAreCountedWhereTheMachineHasThem(t *testing.T) {
 func TestTheWatchReadsEachDriveAsRootWithoutWakingItAndNotesTheFans(t *testing.T) {
 	f := newFake(t)
 	f.server()
-	f.answers["smartctl --json=c -a -n standby /dev/sda"] = Ran{Stdout: ataPassed}
-	f.answers["smartctl --json=c -a -n standby /dev/sdb"] = Ran{Stdout: ataFailing, Status: 24}
-	f.answers["smartctl --json=c -a -n standby /dev/sdc"] = Ran{Stdout: "Device is in STANDBY mode, exit(2)\n", Status: 2}
+	f.answers[smartctlLine("sda")] = Ran{Stdout: ataPassed}
+	f.answers[smartctlLine("sdb")] = Ran{Stdout: ataFailing, Status: 24}
+	f.answers[smartctlLine("sdc")] = Ran{Stdout: strings.Replace(standbyJSON, "/dev/sda", "/dev/sdc", 1), Status: 2}
 	w := NewWatch(f.machine())
 	if err := w.Look(); err != nil {
 		t.Fatal(err)
@@ -298,15 +301,15 @@ func TestTheWatchReadsEachDriveAsRootWithoutWakingItAndNotesTheFans(t *testing.T
 	if _, ok := r.FansRan["nct6775/fan1"]; !ok || len(r.FansRan) != 1 {
 		t.Errorf("the turning fan is noted, the empty header not: %v", r.FansRan)
 	}
-	// Ten minutes have not passed: the fans are looked at, the drives are not read again.
+	// An hour has not passed: the fans are looked at, the spinning drives are not read again.
 	f.now = f.now.Add(FanEvery)
 	f.calls = nil
 	if err := w.Look(); err != nil || len(f.calls) != 0 {
-		t.Fatalf("drives read again within ten minutes: %q %v", f.calls, err)
+		t.Fatalf("drives read again within the hour: %q %v", f.calls, err)
 	}
 	// A new watch (a restart) keeps what the last one read, and a drive asleep keeps its reading before.
-	f.now = f.now.Add(DriveEvery)
-	f.answers["smartctl --json=c -a -n standby /dev/sda"] = Ran{Stdout: "Device is in SLEEP mode, exit(2)\n", Status: 2}
+	f.now = f.now.Add(SpinningEvery)
+	f.answers[smartctlLine("sda")] = Ran{Stdout: strings.Replace(standbyJSON, `"STANDBY"}`, `"SLEEP"}`, 1), Status: 2}
 	w2 := NewWatch(f.machine())
 	if err := w2.Look(); err != nil {
 		t.Fatal(err)
@@ -315,7 +318,7 @@ func TestTheWatchReadsEachDriveAsRootWithoutWakingItAndNotesTheFans(t *testing.T
 	if !r.Drives["sda"].Asleep || len(r.Drives["sda"].Smartctl) == 0 {
 		t.Errorf("sda asleep keeps its reading: %+v", r.Drives["sda"])
 	}
-	ds, _ := f.machine().Drives()
+	ds, _ := f.machine().Drives(f.machine().ReadReadings())
 	if ds[0].Health != "passed" || !ds[0].Asleep || ds[1].Health != "failing" {
 		t.Errorf("drives = %+v", ds)
 	}
@@ -325,7 +328,7 @@ func TestTheWatchNotRootAsksThroughSudoAndSaysWhatStopsIt(t *testing.T) {
 	f := newFake(t)
 	f.uid = 1000
 	f.drive("sda", "X", "ATA     ", 100, true)
-	f.answers["sudo -n smartctl --json=c -a -n standby /dev/sda"] = Ran{Stderr: "sudo: a password is required\n", Status: 1}
+	f.answers["sudo -n "+smartctlLine("sda")] = Ran{Stderr: "sudo: a password is required\n", Status: 1}
 	got := f.machine().ReadDrive("sda", nil)
 	if got.NotRead != "the watch is not root and may not run smartctl through sudo without a prompt" {
 		t.Errorf("not read = %q", got.NotRead)
@@ -342,6 +345,7 @@ func TestTheHealthAnswerIsWhatTheNodeEngineReads(t *testing.T) {
 	f.server()
 	f.file("/sys/class/hwmon/hwmon0/temp1_input", "101000")
 	f.file("/sys/devices/system/edac/mc/mc0/ue_count", "2")
+	f.fresh()
 	var tool func(map[string]any) (any, error)
 	for _, tl := range Tools(f.machine()) {
 		if tl.Name == "sensors_health" {
diff --git a/modules/sensors/cmd/sensors/temperatures.go b/modules/sensors/cmd/sensors/temperatures.go
index a89ceca0..a59909ac 100644
--- a/modules/sensors/cmd/sensors/temperatures.go
+++ b/modules/sensors/cmd/sensors/temperatures.go
@@ -43,6 +43,11 @@ type Temperature struct {
 	Where string `json:"where"`
 }
 
+// DrivesHealth says whether this sensor's critical value makes the module unhealthy. A board chip's
+// (a Super I/O's) is not to be trusted: its inputs and their limits are whatever the firmware left, and a
+// wrongly wired input reads far past them on a healthy machine. It is shown, and raises nothing.
+func (t Temperature) DrivesHealth() bool { return !(t.Source == "hwmon" && t.Kind == "board") }
+
 // Name is how a person reads which sensor this is.
 func (t Temperature) Name() string {
 	what := t.Chip
@@ -67,7 +72,12 @@ func (t Temperature) Name() string {
 // sane says whether a value in °C is a temperature or a threshold the hardware means as one. A
 // threshold given as the format's largest value (an NVMe sensor's 65261.85 °C) or as 0 means none, and
 // so does anything a machine cannot be at and still answer.
-func sane(c float64) bool { return c > -40 && c < 200 && c != 0 }
+func sane(c float64) bool { return reading(c) && c != 0 }
+
+// reading says whether a value in °C is a temperature a machine can be at. 0 °C is one. 127 and 127.5 °C
+// are not: they are what a board chip's input with nothing on it answers (a phantom AUXTIN, the largest
+// value of its eight-bit register), never a real sensor's reading.
+func reading(c float64) bool { return c > -40 && c < 200 && c != 127 && c != 127.5 }
 
 func milli(v int64) *float64 {
 	c := round1(float64(v) / 1000)
@@ -157,7 +167,7 @@ func (m *Machine) hwmonTemperature(dir, chip, stem, drive string) Temperature {
 		t.Kind = "drive"
 	}
 	v, ok := m.readInt(dir + "/" + stem + "_input")
-	if !ok || !sane(float64(v)/1000) {
+	if !ok || !reading(float64(v)/1000) {
 		t.NotRead = "the sensor gave no temperature"
 		if ok {
 			t.NotRead = fmt.Sprintf("the sensor answered %.1f °C, which is no temperature", float64(v)/1000)
@@ -190,7 +200,7 @@ func (m *Machine) zone(dir string) Temperature {
 	t := Temperature{Source: "thermal zone", Chip: typ, Kind: kindOf(typ), Label: strings.TrimPrefix(dir, "/sys/class/thermal/"),
 		Where: dir}
 	v, ok := m.readInt(dir + "/temp")
-	if !ok || !sane(float64(v)/1000) {
+	if !ok || !reading(float64(v)/1000) {
 		t.NotRead = "the zone gave no temperature"
 		if ok {
 			t.NotRead = fmt.Sprintf("the zone answered %.1f °C, which is no temperature", float64(v)/1000)
diff --git a/modules/sensors/cmd/sensors/tools.go b/modules/sensors/cmd/sensors/tools.go
index 1bf8e6d0..2e28bd98 100644
--- a/modules/sensors/cmd/sensors/tools.go
+++ b/modules/sensors/cmd/sensors/tools.go
@@ -33,9 +33,10 @@ func Tools(m *Machine) []stdio.Tool {
 		},
 		{
 			Name: "sensors_fans",
-			Description: "Every fan the machine reports: RPM, the chip's own minimum and alarm where it has them, and whether the " +
-				"module's watch saw it turning since the machine started. STOPPED when its chip says so, or when it ran and reads 0 " +
-				"while a temperature is at or above its own maximum; a fan resting at 0 on a cool machine is not stopped. " +
+			Description: "Every fan the machine reports: RPM, the chip's own minimum and alarm where it has them (said in a note, never " +
+				"flagged: a zero-fan GPU is under its minimum by design and a board chip latches its alarm), and whether the " +
+				"module's watch saw it turning since the machine started. STOPPED only when it ran and reads 0 while the CPU, the " +
+				"board or a sensor on its own chip is at or above its own maximum. " +
 				"Replaces sensors. (r)",
 			Run: func(map[string]any) (any, error) {
 				l := m.LookNow()
@@ -50,7 +51,7 @@ func Tools(m *Machine) []stdio.Tool {
 				"error log and the last self-test. FAILING when the drive says so, HIGH for the early signs. Replaces smartctl -a " +
 				"and nvme smart-log. (r)",
 			Run: func(map[string]any) (any, error) {
-				ds, note := m.Drives()
+				ds, note := m.Drives(m.ReadReadings())
 				out := map[string]any{"drives": ds, "count": len(ds)}
 				if note != "" {
 					out["note"] = note
@@ -71,9 +72,11 @@ func Tools(m *Machine) []stdio.Tool {
 		{
 			Name: "sensors_health",
 			Description: "Whether the hardware is failing, as the module's health check (novox/hq ADR 0256): unhealthy when a " +
-				"temperature is at or above its own critical value, a drive says it is failing, a fan that ran has stopped while " +
-				"the machine is hot, or memory reports an uncorrectable error; with why, one sentence per finding. HIGH readings " +
-				"do not make it unhealthy. (r)",
+				"CPU, GPU, drive or zone temperature is at or above its own critical value (a board chip's is shown, never judged), " +
+				"a drive says it is failing, a fan that ran reads 0 while the CPU, the board or its own chip is at its own maximum, " +
+				"memory reports an uncorrectable error, or the module's watch has not written its readings for five minutes; with " +
+				"why, one sentence per finding. HIGH readings, a fan's alarm bit and a fan under its minimum do not make it " +
+				"unhealthy. (r)",
 			Run: func(map[string]any) (any, error) {
 				return m.LookNow().Health(), nil
 			},
diff --git a/modules/sensors/cmd/sensors/watch.go b/modules/sensors/cmd/sensors/watch.go
index 0d32a1fa..a7eefb6e 100644
--- a/modules/sensors/cmd/sensors/watch.go
+++ b/modules/sensors/cmd/sensors/watch.go
@@ -4,27 +4,44 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"sync"
 	"time"
 )
 
 // The watch: the module's long-running process, run by the node-engine as root (novox/hq ADR 0256). It
-// reads each drive's health every DriveEvery, so the tools, which run as the operator account, need no
-// root; it never wakes a drive in standby; and it notes every FanEvery which fans are turning, so a fan
-// that ran and stopped can be told from one that never runs. It writes ReadingsPath and nothing else.
+// reads each drive's health, so the tools, which run as the operator account, need no root; it never wakes
+// a drive in standby; and it notes every FanEvery which fans are turning, so a fan that ran and stopped can
+// be told from one that never runs. It writes ReadingsPath and nothing else.
 //
-// The module's health is judged on it: by its unit (alive), and by asking sensors_health what the
-// hardware says.
+// **The drives never hold up the fans.** In Run, the drives are read beside the looks at the fans, one
+// pass at a time, each smartctl bounded by CommandTimeout; the file is written every FanEvery whatever a
+// drive does, so a watch whose file grows old is a watch that is hung or gone, and the health check says
+// so (StaleAfter).
+//
+// **A spinning drive is read at most hourly.** Reading SMART from an awake drive can restart its standby
+// timer, so a drive read more often than its timer is set for never spins down. Whether the machines'
+// drives are set to spin down at all is not known to the mesh (no tool reads a drive's standby timer), so
+// the watch reads a rotational drive every SpinningEvery and smartd checks every drive at the same
+// interval (its -i, in the module's drop-in). A drive in standby is never woken: smartctl is asked with -n.
 
 const (
-	FanEvery   = 30 * time.Second
-	DriveEvery = 10 * time.Minute
+	FanEvery = 30 * time.Second
+	// SolidStateEvery is how often a drive that does not spin is read.
+	SolidStateEvery = 10 * time.Minute
+	// SpinningEvery is how often a rotational drive is read: longer than a spin-down timer is commonly set.
+	SpinningEvery = time.Hour
 )
 
 // Watch keeps the readings between looks.
 type Watch struct {
-	M        *Machine
+	M *Machine
+	// Async reads the drives beside the fans instead of before them; Run sets it.
+	Async bool
+
+	mu       sync.Mutex
 	readings Readings
-	lastRead time.Time
+	reading  bool
+	done     sync.WaitGroup
 }
 
 // NewWatch starts from what an earlier watch of this boot left, so a restart neither forgets the fans
@@ -39,48 +56,100 @@ func NewWatch(m *Machine) *Watch {
 		if w.readings.FansRan == nil {
 			w.readings.FansRan = map[string]time.Time{}
 		}
-		for _, d := range r.Drives {
-			if d.ReadAt.After(w.lastRead) {
-				w.lastRead = d.ReadAt
-			}
-		}
 	}
 	w.readings.BootID = m.bootID()
 	return w
 }
 
-// Look is one look: the fans always, the drives when DriveEvery has passed. It writes the file.
+// due lists the drives whose next look has come: never looked at, or looked at longer ago than their
+// cadence.
+func (w *Watch) due(now time.Time) (due []Drive, present map[string]bool) {
+	present = map[string]bool{}
+	for _, d := range w.M.BlockDrives() {
+		present[d.Device] = true
+		every := SolidStateEvery
+		if d.Rotational {
+			every = SpinningEvery
+		}
+		last, ok := w.readings.Drives[d.Device]
+		looked := last.LookedAt
+		if looked.IsZero() {
+			looked = last.ReadAt
+		}
+		if !ok || now.Sub(looked) >= every {
+			due = append(due, d)
+		}
+	}
+	return due, present
+}
+
+// Look is one look: the fans always, the drives that are due. It writes the file.
 func (w *Watch) Look() error {
 	now := w.M.Now()
-	for _, f := range w.M.Fans() {
+	fans := w.M.Fans()
+	w.mu.Lock()
+	for _, f := range fans {
 		if f.RPM != nil && *f.RPM > 0 {
 			w.readings.FansRan[f.Key] = now
 		}
 	}
-	if w.lastRead.IsZero() || now.Sub(w.lastRead) >= DriveEvery {
-		present := map[string]bool{}
-		for _, d := range w.M.BlockDrives() {
-			present[d.Device] = true
-			var before *DriveReading
-			if b, ok := w.readings.Drives[d.Device]; ok && len(b.Smartctl) > 0 {
-				before = &b
-			}
-			w.readings.Drives[d.Device] = w.M.ReadDrive(d.Device, before)
+	due, present := w.due(now)
+	for name := range w.readings.Drives {
+		if !present[name] {
+			delete(w.readings.Drives, name)
 		}
-		for name := range w.readings.Drives {
-			if !present[name] {
-				delete(w.readings.Drives, name)
-			}
-		}
-		w.lastRead = now
 	}
+	start := len(due) > 0 && !w.reading
+	if start {
+		w.reading = true
+	}
+	w.mu.Unlock()
+	if start {
+		w.done.Add(1)
+		if w.Async {
+			go w.readDrives(due)
+		} else {
+			w.readDrives(due)
+		}
+	}
+	return w.write()
+}
+
+// readDrives reads each due drive, one after another, and keeps each reading as it comes.
+func (w *Watch) readDrives(due []Drive) {
+	defer w.done.Done()
+	for _, d := range due {
+		w.mu.Lock()
+		var before *DriveReading
+		if b, ok := w.readings.Drives[d.Device]; ok && len(b.Smartctl) > 0 {
+			before = &b
+		}
+		w.mu.Unlock()
+		got := w.M.ReadDrive(d.Device, before)
+		w.mu.Lock()
+		w.readings.Drives[d.Device] = got
+		w.mu.Unlock()
+	}
+	w.mu.Lock()
+	w.reading = false
+	w.mu.Unlock()
+}
+
+// WaitDrives waits for a pass over the drives that is under way.
+func (w *Watch) WaitDrives() { w.done.Wait() }
+
+func (w *Watch) write() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
 	w.readings.WrittenAt = w.M.Now()
 	return w.M.WriteReadings(w.readings)
 }
 
-// Run looks at once and every FanEvery until ctx ends. A look that cannot write is said on standard
-// error, which the node-engine keeps in the unit's journal, and the watch goes on.
+// Run looks at once and every FanEvery until ctx ends, the drives beside the fans. A look that cannot
+// write is said on standard error, which the node-engine keeps in the unit's journal, and the watch goes
+// on.
 func (w *Watch) Run(ctx context.Context) {
+	w.Async = true
 	t := time.NewTicker(FanEvery)
 	defer t.Stop()
 	for {
diff --git a/modules/sensors/module.json b/modules/sensors/module.json
index 49f141dc..7abe7a4f 100644
--- a/modules/sensors/module.json
+++ b/modules/sensors/module.json
@@ -24,7 +24,7 @@
       "type": "file",
       "path": "/etc/smartd.conf",
       "mode": "0644",
-      "content": "# The mesh's (module sensors, novox/hq ADR 0256): what smartd watches. Written whole at every push;\n# an edit here is overwritten.\n#\n# Every drive smartd can find, with every check (-a). A drive in standby is skipped, never woken to be\n# checked, and the skip is not logged (-n standby,q): a server's spinning drives may sleep. No mail is\n# sent (no -m): what smartd finds goes to the journal, and the module's watch reads each drive's health\n# for the mesh's tools and its health check.\nDEVICESCAN -a -n standby,q\n"
+      "content": "# The mesh's (module sensors, novox/hq ADR 0256): what smartd watches. Written whole at every push;\n# an edit here is overwritten.\n#\n# Every drive smartd can find, with every check (-a). A drive in standby is skipped, never woken to be\n# checked, and the skip is not logged (-n standby,q): a server's spinning drives may sleep. A drive\n# behind a USB bridge that cannot say whether it is asleep is checked anyway by smartd, and may be woken\n# (novox/hq ADR 0256); the watch does not read such a drive. No mail is\n# sent (no -m): what smartd finds goes to the journal, and the module's watch reads each drive's health\n# for the mesh's tools and its health check.\nDEVICESCAN -a -n standby,q\n"
     },
     {
       "id": "smartd-drop-ins",
@@ -37,7 +37,7 @@
       "type": "file",
       "path": "/etc/systemd/system/smartd.service.d/10-mesh.conf",
       "mode": "0644",
-      "content": "# The mesh's (module sensors, novox/hq ADR 0256). Written whole at every push; edit nothing here.\n#\n# smartd runs on every machine the module is assigned to, a virtual machine included, where the package's\n# unit would not start; and it stays up on a machine with no drive it can watch (-q never), where it\n# would otherwise exit, because the mesh judges this service as one that stays up.\n[Unit]\nConditionVirtualization=\n\n[Service]\nExecStart=\nExecStart=/usr/bin/smartd -n -q never\n"
+      "content": "# The mesh's (module sensors, novox/hq ADR 0256). Written whole at every push; edit nothing here.\n#\n# smartd runs on every machine the module is assigned to, a virtual machine included, where the package's\n# unit would not start; and it stays up on a machine with no drive it can watch (-q never), where it\n# would otherwise exit, because the mesh judges this service as one that stays up. It checks each drive\n# hourly (-i 3600), not every half hour: a drive read more often than its standby timer never spins down,\n# and the module's watch reads a spinning drive hourly too.\n[Unit]\nConditionVirtualization=\n\n[Service]\nExecStart=\nExecStart=/usr/bin/smartd -n -q never -i 3600\n"
     },
     {
       "id": "smartd",