Today I logged into my NetApp OnCommand System Manager and got an alert message saying Critical: There are no spare disks. That didn't look too good so I checked the disks on both controllers in my NetApp FAS2020 and sure enough one controller had three spares, while the other controller didn't have any. Well it turns out moving the spares is pretty easy, but you have to do if from the ssh terminal.
aggr status -r
Let's say for the sake of argument that there were multiple spares in the above picture, and the one we want to move is device 0c.00.11. First we need to remove ownership of that device by running the following on the owner:
disk assign 0c.00.11 -s unowned -f
Once that is done, run the following on the other controller you want to move the spare to:
disk assign all
That's it! Now you have moved a spare from one filer to another!