Here is how to stop an external hard disk after it has been idle for a while.

I use external hard disks with Raspberry Pis that are accessed only occasionally. Keeping them spinning all day wastes power and makes noise.

hdparm could not control my disk through its USB adapter. It returned SG_IO: bad/missing sense data. hd-idle worked instead.

Install hd-idle

On current versions of Raspberry Pi OS, install hd-idle with APT:

$ sudo apt update
$ sudo apt install hd-idle

Find the external disk's name:

$ lsblk --nodeps --output NAME,MODEL,SIZE,TYPE

Use the whole-disk name, such as sda, not a partition such as sda1. Check the model and size carefully so you do not target the wrong disk.

Test the disk by asking it to enter standby immediately:

$ sudo hd-idle -t /dev/sda

Replace /dev/sda with your disk. Activity may wake it again immediately, and some disks or USB adapters do not support the command.

Configure hd-idle

Open the configuration file:

$ sudoedit /etc/default/hd-idle

Enable the service and set a 10-minute idle timeout for sda:

START_HD_IDLE=true
HD_IDLE_OPTS="-i 0 -a sda -i 600"

The first -i 0 disables spin-down for every disk. -a sda -i 600 then enables it for sda after 600 seconds of inactivity. The -a value omits /dev/.

For two disks with different timeouts:

HD_IDLE_OPTS="-i 0 -a sda -i 600 -a sdb -i 1200"

Apply the configuration and enable the service at boot:

$ sudo systemctl enable hd-idle
$ sudo systemctl restart hd-idle
$ systemctl status hd-idle

Avoid very short timeouts: repeated spin-up cycles can wear out a hard disk. Ten minutes is a sensible starting point. Disk-monitoring tools such as smartmontools can also keep waking the disk.