Write heads used on exchangeable media (e.g., floppy disks, magstripe cards) differ slightly in position and width due to manufacturing tolerances. As a result, one writer might not overwrite the entire area on a medium that had previously been written to by a different device. Normal read heads will only give access to the most recently written data, but special high-resolution read techniques (e.g., magnetic-force microscopy) can give access to older data that remains visible near the track edges.
Even with a perfectly positioned write head, the hysteresis properties of ferromagnetic media can result in a weak form of previous data to remain recognizable in overwritten areas. This allows the partial recovery of...
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