diff --git a/backend/system/disktype_linux.go b/backend/system/disktype_linux.go
index 8639ae7..0fa502b 100644
--- a/backend/system/disktype_linux.go
+++ b/backend/system/disktype_linux.go
@@ -16,32 +16,107 @@ var errNoBlockDevice = errors.New(
 	"no matching block device found",
 )
 
-// IsRotationalDisk reports whether the block device backing the
-// given path is a rotational (spinning) disk.  Detection uses the
-// Linux sysfs interface at /sys/block/<dev>/queue/rotational.
-// Returns false on any error (assumes SSD).
-func IsRotationalDisk(path string) bool {
-	dev, err := deviceForPath(path)
-	if err != nil {
-		return false
-	}
+// DiskProfile is what the scanner needs to know about the device a
+// library sits on.  Both fields are about the same question — how many
+// reads should be in flight at once — and they answer different halves
+// of it, so they travel together rather than as two probes.
+type DiskProfile struct {
+	// Device is the whole-disk kernel name ("sdb"), or "" when the
+	// path could not be resolved to one.
+	Device string
 
-	rotational, err := os.ReadFile(
-		filepath.Join(
-			"/sys/block", dev, "queue", "rotational",
-		),
-	)
-	if err != nil {
-		return false
-	}
+	// Rotational is /sys/block/<dev>/queue/rotational: true for a
+	// spinning disk, where a seek costs milliseconds.
+	Rotational bool
 
-	return strings.TrimSpace(string(rotational)) == "1"
+	// QueueDepth is /sys/block/<dev>/device/queue_depth — how many
+	// commands the drive will accept and reorder at once.  This is
+	// NCQ: a SATA disk with it enabled reports 31 or 32, and one
+	// without reports 1.  Zero means the file was not there to read,
+	// which is the case for anything that is not a SCSI/SATA device
+	// (NVMe, MMC, device-mapper, loop, a VM's virtio disk).
+	//
+	// It is the difference between concurrency helping and hurting.
+	// With queueing, several outstanding reads let the drive service
+	// them in the order its head passes over them, which is most of
+	// why a parallel scan is faster at all.  Without it, every extra
+	// worker is one more seek competing for one head, and the scan
+	// gets slower the harder it is pushed.
+	QueueDepth int
 }
 
-// deviceForPath resolves a filesystem path to its underlying block
-// device name (e.g. "sda") by matching the device major:minor
-// from stat(2) against /sys/block/ entries.
-func deviceForPath(path string) (string, error) {
+// Queues reports whether the drive can reorder outstanding commands.
+//
+// An unknown depth (0) counts as queueing: everything that does not
+// publish this file is a device where concurrency is fine — NVMe has
+// its own queues, virtio and device-mapper are not the physical layer
+// at all.  The only case worth being careful about is the one that
+// says so explicitly.
+func (p DiskProfile) Queues() bool {
+	return p.QueueDepth != 1
+}
+
+// IsRotationalDisk reports whether the block device backing the
+// given path is a rotational (spinning) disk.  Returns false on any
+// error (assumes SSD).
+func IsRotationalDisk(path string) bool {
+	return ProfileForPath(path).Rotational
+}
+
+// ProfileForPath describes the device backing a filesystem path.  A
+// path that cannot be resolved yields the zero profile, which reads as
+// "not rotational, queueing" — the permissive answer, since assuming a
+// spinning disk on an SSD would halve a scan for nothing.
+func ProfileForPath(path string) DiskProfile {
+	dev, err := diskForPath(path)
+	if err != nil {
+		return DiskProfile{}
+	}
+
+	return DiskProfile{
+		Device:     dev,
+		Rotational: sysfsInt(dev, "queue", "rotational") == 1,
+		QueueDepth: sysfsInt(dev, "device", "queue_depth"),
+	}
+}
+
+// sysfsInt reads one small integer out of /sys/block/<dev>/<parts...>,
+// returning 0 when it is absent or unparseable.  Every attribute here
+// is optional: sysfs layout varies by driver, and a missing file is
+// "this device does not say", never an error worth propagating.
+func sysfsInt(dev string, parts ...string) int {
+	p := filepath.Join(
+		append([]string{"/sys/block", dev}, parts...)...,
+	)
+
+	data, err := os.ReadFile(p) //nolint:gosec // sysfs, name from the kernel
+	if err != nil {
+		return 0
+	}
+
+	n, err := strconv.Atoi(strings.TrimSpace(string(data)))
+	if err != nil {
+		return 0
+	}
+
+	return n
+}
+
+// diskForPath resolves a filesystem path to the *whole disk* backing
+// it — "sdb" for a file on "sdb3".
+//
+// It goes through /sys/dev/block/<major>:<minor>, which the kernel
+// maintains as a symlink to the device's own sysfs directory, and then
+// walks up to the parent when that directory turns out to be a
+// partition.  The previous implementation scanned /sys/block comparing
+// dev numbers and, failing an exact match, took the first entry whose
+// *major* agreed — and every SATA disk shares major 8.  So a library on
+// /dev/sdb3 resolved to whatever /sys/block listed first, which is
+// alphabetical, which is sda.  On the machine this was found on that
+// meant a 6 TB spinning disk was read as the SSD next to it and scanned
+// with one worker per core.  Matching on major alone cannot be right
+// whenever a machine has two disks, which is the case this exists for.
+func diskForPath(path string) (string, error) {
 	var st syscall.Stat_t
 	if err := syscall.Stat(path, &st); err != nil {
 		return "", fmt.Errorf(
@@ -49,48 +124,50 @@ func deviceForPath(path string) (string, error) {
 		)
 	}
 
-	// Extract major and minor device numbers.
-	major := (st.Dev >> 8) & 0xff
-	minor := st.Dev & 0xff
+	// Linux packs dev_t as 12 bits of major and 20 of minor, split
+	// across the word.  Masking the low byte of each — which is what
+	// this used to do — is right only for the first 256 of either.
+	major := unixMajor(uint64(st.Dev))
+	minor := unixMinor(uint64(st.Dev))
 
-	// Scan /sys/block/ for a matching device.
-	entries, err := os.ReadDir("/sys/block")
+	link := filepath.Join(
+		"/sys/dev/block",
+		strconv.FormatUint(major, 10)+":"+
+			strconv.FormatUint(minor, 10),
+	)
+
+	target, err := filepath.EvalSymlinks(link)
 	if err != nil {
 		return "", fmt.Errorf(
-			"could not read /sys/block: %w", err,
+			"%w: %s (%w)", errNoBlockDevice, link, err,
 		)
 	}
 
-	majorStr := strconv.FormatUint(major, 10)
-	devStr := majorStr + ":" +
-		strconv.FormatUint(minor, 10)
+	// A partition's directory sits inside its disk's, and only the
+	// disk carries `queue`. Climb at most one level: sysfs nests a
+	// partition exactly one deep under its disk.
+	name := filepath.Base(target)
 
-	for _, entry := range entries {
-		devFile := filepath.Join(
-			"/sys/block", entry.Name(), "dev",
-		)
-
-		data, err := os.ReadFile(devFile)
-		if err != nil {
-			continue
-		}
-
-		content := strings.TrimSpace(string(data))
-
-		if content == devStr {
-			return entry.Name(), nil
-		}
-
-		// The filesystem might be on a partition (e.g. sda1)
-		// whose parent block device is sda.  Check if the
-		// major number matches.
-		parts := strings.SplitN(content, ":", 2)
-		if len(parts) == 2 && parts[0] == majorStr {
-			return entry.Name(), nil
-		}
+	if _, err := os.Stat(filepath.Join(target, "queue")); err != nil {
+		name = filepath.Base(filepath.Dir(target))
 	}
 
-	return "", fmt.Errorf(
-		"%w for %s", errNoBlockDevice, devStr,
-	)
+	if name == "" || name == "." || name == string(filepath.Separator) {
+		return "", fmt.Errorf(
+			"%w for %d:%d", errNoBlockDevice, major, minor,
+		)
+	}
+
+	return name, nil
+}
+
+// unixMajor and unixMinor decode a Linux dev_t.  Spelled out rather
+// than taken from golang.org/x/sys/unix so this file stays readable
+// beside the encoding it is undoing.
+func unixMajor(dev uint64) uint64 {
+	return (dev>>8)&0xfff | (dev >> 32 & ^uint64(0xfff))
+}
+
+func unixMinor(dev uint64) uint64 {
+	return dev&0xff | (dev >> 12 & ^uint64(0xff))
 }
diff --git a/backend/system/disktype_other.go b/backend/system/disktype_other.go
index f5a59f1..f1f1b25 100644
--- a/backend/system/disktype_other.go
+++ b/backend/system/disktype_other.go
@@ -2,9 +2,34 @@
 
 package system
 
+// DiskProfile is what the scanner needs to know about the device a
+// library sits on.  See the Linux implementation for what each field
+// means; off Linux nothing fills them, because neither macOS nor
+// Windows publishes an equivalent of sysfs's `rotational` and
+// `queue_depth` without going through platform APIs this package
+// deliberately does not link.
+type DiskProfile struct {
+	Device     string
+	Rotational bool
+	QueueDepth int
+}
+
+// Queues reports whether the drive can reorder outstanding commands.
+// Always true here: an unknown depth is the permissive answer, and
+// assuming otherwise would halve every scan on every Mac.
+func (p DiskProfile) Queues() bool {
+	return p.QueueDepth != 1
+}
+
 // IsRotationalDisk reports whether the block device backing the
 // given path is a rotational (spinning) disk.  On non-Linux
 // platforms this always returns false (assumes SSD).
 func IsRotationalDisk(_ string) bool {
 	return false
 }
+
+// ProfileForPath describes the device backing a filesystem path.  Off
+// Linux that is the zero profile, which reads as "an SSD that queues".
+func ProfileForPath(_ string) DiskProfile {
+	return DiskProfile{}
+}