While trying to use a  TK50 tape in my MV3600's TK70 drive, the drive
went into read-only mode, and tar complained that it couldn't write to
the tape. This happened on a number of tapes, even though I eventually
found one that I could write to.
Is this happening because the tape was previously written on a TK50
drive??  If so, can the timing track be erased with a powerful permanent
magnet (and from which side?) or is an alternating magnetic field
needed? I'm asking about the permanent magnet because the audio/video
equipment dealers I've talked to don't stock a *good* bulk tape
eraser...

TIA...

Stan