hd-idle — Spin down external disks after a period of idle time. More information: <https://manned.org/hd-idle>

Start a service to manage hard drive spin down. By default hard disks will spin down after 10 minutes of inactivity

systemctl start hd-idle

Spin down a disk immediately

hd-idle -t {{/dev/sdX}}

Set disks to never spin down, then set explicit idle times (in seconds) for disks which have "sda" or "sdb" in their device name

hd-idle -i 0 -a /dev/sda -i {{300}} -a /dev/sdb -i {{1200}}