I noticed that my external hard drive, which uses a cheap SATA to USB enclosure, was constantly spinning, even when I hadn’t accessed it in hours.

I decided to lookup how to coax the drive to spin down.

A little searching showed that you can check a drive’s status and set its idle timeout policy with the hdparm command.

$ sudo hdparm -C /dev/sdb
 drive state is:  active/idle

Setting the timeout is a little odd, the value 120 means 10 minutes. See the man page for details.

$ sudo hdparm -S 120 /dev/sdb 
 setting standby to 120 (10 minutes)

I also read that some drives don’t obey hdparm, so you may want to checkout hd-idle if your drive won’t idle.