“hdparm -S240 /dev/sdX”
At the CLI tells device X to spin down after 20 minutes.
The time is a multiplication of 240 x 5 = 1200 seconds = 20 minutes.
There is an explanation here:
I think hdparm is part of the standard Rockstor package.
Will have to be set for every drive after each reboot, which can be dificult to automatize as drives have a tendency to change letters upon reboot.
I do it manually after each reboot, which thankfully is relatively seldom for my system.