You can manually put a USB HDD that doesn’t automatically spin-down after inactivity to sleep with the following command:
sg_start 0 --pc=3 /dev/sdc
where I assumed your HDD was at
You can determine which
/dev/sd* you’re at with the command
The HDD should automatically spring back to active spinning mode when you write a file to it, etc.
You can install the
sg_start program via:
sudo apt install sg3-utils
The reason you might want to do this include that the HDDs put out some substantial heat when they’re not spun down. You don’t want to spin up/down too frequently (like every 5 minutes) to avoid excessive wear. If you’re not using the drive for the night or a few days, it might be worthwhile to spin them down if heating is an issue.