Final update on this topic, and i’m only going to give the TLDR version. I’m fully satisfied that the way the power settings are configured in OSMC are the least bad settings available. Most drives should spin down just fine (after 20 min of inactivity) and the ones that don’t it is a matter of debate on if the drive or Debian to blame (it is probably the drive).
With my particular drives I was testing (WD MyBook and Easystore) the ones that were never messed with sleep just fine with a nice blinking light to let me know they are asleep. However once I used WD Drive Utilities to change the sleep time the drive presented itself, and interacted with the OS, in a very poor fashion. Unfortunately this is also not easily (if at all) reversible (thanks WD). The solution for this particular drive was to go back to WD Drive Utilities and turn off the sleep settings. This does not get the drive back to a blinking light (sleep) but it does get the drive to spin down perfectly, and that is close enough to the same thing for all practical purposes. With the drive set to any other setting the sleep function was not reliable.