It's possible to enable spinning down of mechanical hard drives using the "Put hard disks to sleep when possible" option in Energy Saver, and it's possible to customize the timeout period with
sudo pmset -a disksleep MIN.
However, if I understand the man pages correctly, this applies to all hard drives connected to the system. Is it possible to only target one HDD (ideally, by UUID)? I have no objection to installing a 3rd party app to accomplish this functionality.