Charlieman said:

And the medium is so contrary. I sort of believe that Gorgonops' solution might work for floppy disks. I'm eyeing some magnets retrieved from a hard disk head. But I know people who used a magnetic bulk eraser (designed for tape media) on Zip and Jaz cartridges which never worked again.

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I think both Zip and Jaz have embedded servo positioning information encoded magnetically on the disk. (Remember, they're sort of floppy/hard disk hybrids.) It generally takes special equipment to write said info, the drives can't do it, so, yeah, not surprised at all that using a bulk eraser would kill those. Floppies don't have that problem.

We used to have lying around the office a really scary machine designed to wipe hard disks magnetically; it came with warnings about leaving unsecured objects close to it, and drives coming off it would be warm to the touch following the ordeal. Drives erased with that thing were likewise *permanently* destroyed.