We had an HA system with nodes TEST-2020-A and TEST-2020-B but A failed. We replaced with with a spare head but all disks that A owned are unavailable to be owned by the new head or by B.
I have tried some of the disk remove_ownership commands but keep getting errors that the disk is not owned by the node.
What do I need to do here for Data OnTap 7.3.4 to get the disks reassigned so I can bring the aggregate online?
If memory serves you boot the new head into the maintenance menu at boot and you can re-assign the disks from there. Make sure you're not in failover of course. From there you can re-assign disk at will.
Richard is correct. This is documented in the Data ONTAP 7.3 Active/Active Configuration Guide, page 187. Here is a direct link to the HTML doc.