How to use FreeBSD camcontrol epc command with USB...

Below is a practical checklist you can follow to make camcontrol epc(8) work on a SATA hard-disk that sits behind a USB bridge on FreeBSD.

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1 Check that the bridge really passes ATA commands

camcontrol (and SMART, APM, EPC, …) only work if the USB-to-SATA bridge implements SAT-2/SAT-3 ATA PASS THROUGH (12/16).
A fast way to test is:

sh
# Identify the drive through the SCSI layer
camcontrol identify /dev/da0

• If you see a normal IDENTIFY-DEVICE block, the bridge is translating OK.
• If you get “unsupported field in SCSI command / ILLEGAL REQUEST”, the bridge does not forward ATA commands – you are out of luck with camcontrol on that enclosure. Many recent ASMedia, JMicron, Realtek, Cypress and Initio bridges work; older or very cheap ones often don’t. Smartmontools

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2 Locate the right device node

USB disks appear as daX (and a matching passY) rather than adaX.
Either node works, but passY makes it absolutely clear you are talking to the CAM pass-through layer:

sh
# List all CAM devices
camcontrol devlist
# Example result
<ST4000DM004-2CV104>  at scbus2 target 0 lun 0 (pass4,da0)

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3 Does the drive support EPC at all?

sh
camcontrol epc /dev/da0 -c status

• Success ➜ the firmware replied with the current EPC state/timers.
• “SETFEATURES EXTENDED POWER CONDITIONS … ABRT” ➜ the disk does not implement EPC (it may only have APM). The FreeBSD Forums

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4 Enable EPC (only needed once per power-cycle)

sh
# Turn APM off first – spec says APM and EPC are mutually exclusive
camcontrol apm /dev/da0      # no number = disable

# Enable the EPC feature set
camcontrol epc /dev/da0 -c enable

If that works you are ready to program timers or disable individual low-power states. Example recipes below come straight from real-world usage and the FreeBSD community docs Server Fault:

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5 Typical things you may want to do

Goal	Command	Notes
See current timers / allowed states	camcontrol epc da0 -c status	Shows which EPC timers are running and which states (Idle _a/b/c, Standby _y/z, DevSleep) the drive may enter.
Disable head-parking but keep spin-down	<br>camcontrol epc da0 -c state -d -p Idle_b<br>camcontrol epc da0 -c state -d -p Idle_c	Use -s to make it persistent.
Disable spin-down completely	camcontrol epc da0 -c state -d -p Standby_z -s	-d = disable that state, -s = save to disk NVRAM so it survives power-cycling.
Change a timer (e.g. Standby_z after 1 h)	camcontrol epc da0 -c timer -p Standby_z -T 3600 -e -s	-e first enables EPC if it was still off. -T is in seconds; fractional values allowed (e.g. 60.5).
Instantly put the drive into Standby_z (for scripts)	camcontrol epc da0 -c goto -p Standby_z	Useful when you want to spin a disk down right now.

(Run the commands as root; they are safe for data but a typo can still leave a drive unwritable until the next reset.)

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6 Persisting the settings

• All EPC state or timer sub-commands accept -s – that asks the firmware to copy the current value into its “saved” page so it survives a power loss.
• Some consumer drives ignore -s; simply re-run your chosen commands from /etc/rc.local or a periodic script at boot.

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7 Troubleshooting cheat-sheet

Symptom	Likely cause / fix
camcontrol: ATA SETFEATURES… ABRT immediately	Disk does not support EPC – stay with APM instead.
Sense key ILLEGAL REQUEST from umass	Bridge does not pass ATA commands ➜ try a different enclosure.
Works on SATA but not on USB	Same as above – or the bridge only supports 12-byte pass-through. FreeBSD uses 16-byte by default; rebuild kernel with CAM_ATA_PASS_THROUGH_12 or test with smartctl -d sat,12 to confirm.
Settings work until reboot only	Add -s, or run the commands from an rc script.

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8 Take-aways

• camcontrol epc works fine through USB as long as the bridge is SAT-compliant.
• Use the daX/passY node, enable EPC once, then configure timers or disable unwanted states with -c timer / -c state.
• Save with -s if the drive firmware supports it; otherwise replay the commands at boot.

After you have verified that your enclosure passes ATA commands, the examples above should let you tune or disable any power-saving state on the drive just as if it were plugged into a native SATA port.

Why FreeBSD prints

camcontrol: The epc subcommand only works with ATA protocol devices

camcontrol epc refuses to run unless the CAM layer thinks the target speaks the ATA protocol.
When a disk is behind a USB–-to-SATA adapter the kernel sees it as a SCSI device (daX).
If (and only if) the bridge advertises SAT (SCSI → ATA Translation) CAM marks the device internally as PROTO_ATA, so camcontrol epc, apm, SMART, etc. work.
If the bridge does not expose SAT CAM marks it PROTO_SCSI, and epc bails out with the message you are seeing. Wikipedia The FreeBSD Forums

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Checklist to find (and fix) the problem

Step	Command	What you should see	What it means
1. Does the kernel think it’s ATA?	camcontrol identify /dev/da1	A normal IDENTIFY block	Bridge is SAT-compliant → jump to step 3
		passN: <…> SPC-4 SCSI device or “ATA IDENTIFY failed”	Bridge is not SAT-compliant → you cannot use epc, go to Only real fix below
2. Double-check with smartctl	smartctl -d sat -i /dev/da1	Drive info prints	Confirms SAT works
	smartctl -d sat,12 -i /dev/da1 prints but plain -d sat fails	Bridge only accepts 12-byte ATA PASS-THROUGH – see Old bridges below FreeBSD Manual Pages
3. Now ask EPC again	camcontrol epc /dev/da1 -c status	EPC state/timers	Success – you’re done

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If it still fails even though identify works

1. Old bridges that only support 12-byte pass-through

   FreeBSD uses the 16-byte SAT commands by default.
   Re-build your kernel with

   options CAM_ATA_PASS_12
   

   or, on 14-CURRENT and newer, add to /boot/loader.conf:

   kern.cam.ata.pass_12=1
   

   Then reboot and repeat the test. FreeBSD Manual Pages

2. Ancient userland / kernel

   EPC support for SAT devices was completed in FreeBSD 13.0.
   Make sure you’re on at least 13.2-RELEASE or any 14-STABLE snapshot.

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Only real fix when the bridge is not SAT-compliant

If step 1 already shows a plain “SPC-4 SCSI device”, your enclosure does not forward ATA commands.
There is no software workaround – camcontrol epc, SMART, APM, and friends will never work through that adapter.
You need a USB-SATA bridge that advertises SAT (look for chips such as ASMedia ASM1153/1153E, ASM235CM, JMicron JMS567/JMS578, Realtek RTL9210) or plug the disk into a native SATA port. The FreeBSD Forums

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Quick summary

• The error simply says “CAM does not see an ATA device here”.
• Most of the time that means your USB enclosure doesn’t implement SAT; replace it.
• If the bridge is SAT but only handles 12-byte commands, enable CAM_ATA_PASS_12 or update to a current kernel.
• Once the bridge passes ATA, camcontrol epc works exactly as it does on a direct SATA port.

Run the three-step checklist above and you’ll know immediately which case you have and how to proceed.