Hdparm: https://wiki.archlinux.org/title/Hdparm

E.g:

hdparm -B 127 /dev/sda

I know it's archwiki, but it's the same on every distro

Wow, this question takes me back to like the 00's when laptops had battery life measured in minutes.

Don't do that

You can use hdparm with the -S parameter to set the standby/spindown time for a hard drive.

https://wiki.archlinux.org/title/Hdparm#Power_management_configuration

umount maybe

Spin down when not in use is normally default behaviour for most systems already, you don't need to do anything.

My external HDD automatically does this regardless of OS, it is an Orico. I have 2 of them with a 2tb HDD in one and a 8tb in another. These are only used for playback and I believe the spin down after 30 sec of no activity