Nicholas -

Utilizing a utility such as tapeutil's rwtest function, exercise the
supposedly bad tape to evaluate whether it really is, or if just one
drive is acting up.  I would additionally physically inspect the tape,
pulling out a meter or so of it to have a look: in some cases, a drive
has a mechanical defect and is chewing up tapes - where it will mangle
all your tapes if given the opportunity.  (In 3590 experience, I had a
case where the CE extricated a "bad tape" from the drive and left it
for me - where the tape's leader block was missing... which begs the
question of where it was: still inside the drive, causing mayhem with
other inserted tapes.)

   Richard