This is a "How we fixed it" rather than a question - hope it helps somebody.
One of our sites - 2 Node Clustered NW6.5 SP5 w/shared storage, BENW 9.1
Build 1158 running in Media Management mode on a cluster resource - replaced
a failing LTO2 drive, which then would not work. Got the above errors during
test backups. Staff at site rebooted the server a few times before calling
me. Tried the usual reseat of cables and SCSI Adapter. HAM drivers were the
same as at other working sites, as was firmware.
I did a BESTOPC, then a BESTARTC -CP, which loads BE cluster-aware and forces
Partition Management Mode. Test backup worked. Unloaded it, ran BESTARTC
-CM, which put it back into MM Mode. Still worked, and works to this day.