Hdparm - Checking if a drive is spun down

Is there anyway to find out if a hard drive has spun down? I've been using the following command to spin it down after idle time:

hdparm -S 120 /dev/hdX

However I'm not sure it's working correctly and was wondering how to check the current power state to see if has correctly powered down.

You can find out the power status with the -C flag to hdparm:

hdparm -C /dev/hdX

As explained by man 8 hdparm:

-C

Check the current IDE power mode status, which will always be one of unknown (drive does not support this command), active/idle (normal operation), standby (low power mode, drive has spun down), or sleep‐ ing (lowest power mode, drive is completely shut down). The -S, -y, -Y, and -Z flags can be used to manipulate the IDE power modes.

As I don't have hdparm nor smartctl available on my NAS I checked the /proc as I assumed that it had to be there somewhere, too.

Well, if it's down:

root@NAS:~# cat /proc/d_suspdtime
SuspendingTime = 15
Disk0: STANDBY
Disk1: No_Disk
root@NAS:~# 

If the kitten would go crazy on it:

root@NAS:~# cat /proc/d_suspdtime
SuspendingTime = 15
Disk0: ACTIVE
Disk1: No_Disk
root@NAS:~# 

I also got

root@NAS:/proc# cat /proc/d_suspdtime
SuspendingTime = 15
Disk0: NO_ACTIVE
Disk1: No_Disk
root@NAS:~# 

Guessing from some testing:

• ACTIVE: the HDD did something within a certain timespan (it doesn't seem to mean that it does something right now)
• NO_ACTIVE: there was some activity, but in a while it'll be going to standby
• STANDBY: well, the HDD is in standby

And to be complete on this SuspendingTime is the time in minutes after the HDD should go to standby.

Since I'm no Unix-pro I can't tell if you are supposed to have /proc/d_suspdtime on your system, too (guessing from Google results it may be related to NAS or busybox).