I have a noisy hard drive I rarely use and want to silent. I would like to put it in sleep mode after some time (20 sec) when it's not mounted on the system.
The gnome-disks
utility shows a "standby timeout" option:
The hard drive never gets to sleep automatically; I can manually put it to sleep/wake it up with the gear menu:
Why does my drive doesn't automatically go to sleep? How can I make it go to sleep automatically after no activity?