Hallo,
i have the same problem with STK L700 and IBM LTO1 - Drives. I have read one solution could be to add the line MEDIA_UNMOUNT_DELAY = 1200 to the bp.conf in /usr/openv/netbackup/ but it solved not the problem. I pressume it could be a Problem with the SAN. I used a portdisable port/ portenable port on our brocade switches, this solved a few problems with some drives. But sometimes we see the problem appears once more. We do in a short period of time a update of the brocade firmware and the firmware of the drives/Libary, i am interested to see, if it helps...
regards