Manually put a USB HDD that doesn’t automatically spin-down after inactivity to sleep with sg_start

apt install sg3-utils

Determine which device ID by:

lsblk

Assuming the HDD to spin down was at /dev/sdc:

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.