After a long time banging my head against the walls (metaphorically) trying to solve this I finally found the culprit with the help from fatrace util (it's in Debian repos), along with btrace.
The culprit was udisks2. After stopping udisksd the disks spun…
TL;DR version: btrace suggests the culprit is a "[pool]" accessing the disk every 600 seconds. What is it and how do I stop it?
I've been using OMV for a while and I had this working before, but at some point it stopped working (probably…