The idea was that, since the drive head doesn’t pass over precisely the same area on the track every time it writes to the disk, the track has to be wide enough to account for this lack of precision. Thus, a single overwrite pass could leave a portion of the track’s edge unchanged and readable with an electron microscope or something.
I always figured that a write wouldn't change the magnetic field 100%. Say it's 90% effective at changing the magnetic field. There's now four options:
