I haven’t used hd-idle before but know of it. The UUID’s you mention are all btrfs related. You need to use something that references the base drive, not any filesystem related stuff on top of it. Try using the /dev/disk/by-id names for example.
Another method you could try for configuring idle power down is via smartctl options. I have read that these, like hd-idle, can sometimes work when hdparm does not.
We have an open issue which details extending our custom smart options to encompass these smartctl issued hd power down settings:
In the issue the example given is for smart through a raid controller as this is a typical scenario for hdparm not working but it’s worth a try on straight drive access, if this is what you have.
Could it be possible that my problem belongs to Rockstor and btrfs?
I´ve gone back to OMV now because of less time to handle that PWM Problem besides work and family but Rockstor is that much clean and easy that i would like to solve my problem.
So i´ve installed Rockstor again on my USB SLC Stick and guess what? I have absolutly no problem to put the OMV EX4 HDDS which are unmounted to standby. Even with hdparm -Y it works like a charm.
So i think the problem belongs to Rockstor.
I´ve used a single Pool during my last try because i don´t need anything else.