RAID-5 inoperable, but no disk failure
Solution 1:
I think the u1 disk is a leftover from another RAID5 array, so the controller detect it but can clearly not activate it. To use that disk as a spare, you need to first clear the RAID config on u1 disk and then re-add that disk to the array (as a spare).
Before doing anything, be sure to study the controller docs and to have current, valid backups