Hi there,
ONTAP 8.1 would require you to run commands on the node owning the disk at the time, so potentially two SSH sessions.
If you log in to the controller with the prefailed disk, and run "aggr status -r", I assume that there are no spare disks listed and the pre-failed disk is still part of the aggregate?
To begin - if you plan to run this controller longer term, you should download this document - https://library.netapp.com/ecm/ecm_download_file/ECMP1141781 - we no longer support ONTAP 8.1 and the documentation may be removed at any time.
On the controller with the spare disks, follow the procedure on page 43 - turn off disk auto assign and remove_ownership of the spare disk. Then on the controller that needs the disk, run "disk show -an", and verify the spare disk is visible and unowned, then run "disk assign <diskname>". At that point it should automatically recognise it as a spare, but I can't remember the logic for 8.1 - it may not automatically replace it since it wasn't present when the disk failed, so then yes, run the "disk replace" command.
Good luck!