The whole point of multiple write passes is that the physical head does not write a track at the same exact physical location every time. With the right equipment ( disk manufacturers and data recovery experts have it), you can center the read head off to the side of the track, and read a value from a previous write. The accuracy of the read depends on the normal your mileage may vary factors, how long the previous value was there , where the head was previously centered (within the limits of being able to read correctly) etc. If you write random bit patterns, multiple times ( and different bit patterns on each pass), then the difficulty of extracting the previous data increases. How much the difficulty increases is known by the people who do this for a living (NOT me).
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