I have just come across this issue as well. Scenario: Had a couple of Clustered ONTAP nodes that we needed to re-purpose. The idea was to go to maintenance mode and remove all disk ownership and then just assign the disks.
However I had the message "ownership calculation is releasing reservation on disk " A disk show -n showed all disks as unowned but any attempt to own them would result in a message telling me that the disk was owned by another.
Following Franja.Schmid info above, I took the chance to firstly power off the shelves and head and then power them back on. Problem Solved
If that doesen't do it, then, >disk assign <disk_id> -f
You also might need to go to advanced mode before issuing those commands. >priv set advanced.
>priv set will simply bring you back to normal mode.
Do you know where that disk came from?
I mean, if it's not a empty one and there is a part of aggregate in it, and you assign it to your filer, you will get a warning message. Aggregate in degraded mode, or something like that, so don't panic, you just have to destroy it then