I had a similar problem with a DAT72 drive.
Apparently I had fixed the problem with a service pack update, but the symptoms didn't go away.
I had to erase all the tapes which removes all the markers.
You may have already fixed the problem but just need to erase that tapes.
Of course it could be something completely different but I thought I would mention what I did in my case.
Thanks,
Niall.