How do I spin down a secondary disk when not mounted?
Solution is a terminal command with which you can control the sleep timer.
I set it to 1 minute.

sudo pmset -a disksleep VALUE


This still means the disk wakes up every hour for the time machine backup, and also every 15 minutes-ish for god knows what reason.

But its much better than before, just annoying that it regularly wakes up for a minute, which is distracting.