The MFM is a specialized version of an atomic force microscope (AFM), and has no relationship to the electron microscope (SEM). I used to design and operate some of the first high sensitivity MFMs in the 90's, and we looked at many hard disks sent to us by all of the major HDD manufacturers. Bottom line is, when overwriting a single track, you can see the bits from the previous data due to head alignment errors when rewriting the track. Rewriting a whole disk may be a different story as the tight track packing will most likely squeeze out any old data left between the tracks.

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