On a machine with TrueNAS Core (TrueNAS-13.0-U6.1), we have a large JBOD attached. When trying to add the drive as a hotspare, TrueNAS shows a list of four drives, three of which are already part of a pool.
e.g. da77
is clearly part of a pool:
If there wasn’t important information on the pool I would just try and see what happens .
Not sure if this is relevant information but da77
is the currently active connection in the multipath and the new drive intended as hot spare does not appear in the list, even though it is just another drive in the same JBOD.
Edit: We have tested this on a different but very similar machine and the issue does not appear there.