Before you declare that old TK50 tape useless and un-formattable (or un-INITable), give this a go:
How to format a TK50 tape using a magnet
in 13 easy steps
An illustrated manual for the mechanically challenged
What you’ll need:
1. An unreadable TK50 (Compactape) tape
2. A large, strong magnet, or a bunch of small strong magnets
3. Something to help you unlock the tape; I used three of those old hinges:
Step By Step Procedure
Step 1: Observe the first tape movement lock
Step 2: Insert the hinge into the lock so that the plastic lever inside is pushed aside
Step 3: Observe the second tape movement lock
Step 4: Insert the hinge into the lock so that the plastic lever inside is pushed aside
Step 5: Observe the tape door lock
Step 6: Press the plastic latch downwards and the door will pop open
Step 7: Place an object inside between the tape door and the cassette body so the door would not snap shut
Step 8: Pull out the tape leader; if it is stuck, check that the two locks are properly unlocked
Step 9: Pull out about 2 feet of tape, hold one side and make several passes with the magnets along the tape
Step 10: Flip the tape on its back
Step 11: Press the tape reel firmly in (but not all the way), and reel in the tape by slowly rotating counter-clockwise
Step 12: The tape leader should be in its original, fully-wound position
Step 13: Take out all the locks and you’re done !
If you still cannot /INIT the tape, you can try and repeat the procedure with a stronger magnet, or add it to your collection of dead tapes.