Manually put a USB HDD that doesn’t automatically spin-down after inactivity to sleep with
apt install sg3-utils
Determine which device ID by:
Assuming the HDD to
sg_start 0 --pc=3 /dev/sdc
The HDD wakes up automatically when writing/reading a file on it.
We spin down hard drives because HDDs put out substantial heat when spinning, but don’t spin up/down too frequently to avoid excessive wear.