Hi, I recently experienced the same issue here. Until Debian Stretch everything was fine, but I recently upgraded to Debian Buster (pre-release). I previously just set the spindown time and this used to work fine. Hdparm -Y on the devices (using the plain /dev/sd? paths) still worked, but automatic spindown was somehow broken. I tried enabling APM (setting it to 127) but that didn't change a thing - I previously only set spindown time and that worked just fine. I then switched to /dev/disk/by-id/ paths, that didn't work either.
What I have noticed, however, is that spindown_time settings up to 59 work; anything from 60 (5 minutes) and up won't. Hope this helps. Cheers Stijn
