Just before it was decommissioned a ten year old MRI machine, we very, very, very carefully used it to erase a bunch of magnetic media based storage devices. Hard drives, floppies, mag tape. In the really old days we would toss the drives off of the third story balcony to a concrete loading dock. Go down and pick them up and shake them. If they rattled they were done. If not, repeat process until they did

BTW, the old dot matrix printer heads were great for erasing floppies and credit card mag stripes. Just line them up on the shield and run off some print outs. The pin firing mechanism applied a very strong alternating field. If you still have access to one, just put a compass on the paper shield and watch it spin/twitch as the print head scans back and forth.