Radio Shack used to sell a "Bulk Tape Eraser". I gave mine to the
college.
Those are on eBay, and even Amazon.
About 25 years ago, Radio Shack/Tandy changed the label and box, and
called it "Bulk Disk Eraser".
The college bought one, and discarded mine.
But, as everyone knows, the one SURE way to totaally erase a disk is to
store the only copy of something irreplaceable on it.
--
Grumpy Ol' Fred cisin(a)xenosoft.com