[Dshield] Secure Data Removal

Frank Knobbe frank at knobbe.us
Fri Jun 4 19:40:08 GMT 2004


On Fri, 2004-06-04 at 10:34, Josh Tolley wrote:
> When you overwrite a hard drive with all 0's, you change the strength of 
> the magnetism of each bit on the drive to something less than a 
> particular threshold.

More important than remnant magnetism is the fact that the head of a
hard drive is not exactly positioned over a given track. If the head
moves in on that track from the other side, it will be slightly off than
where it was positioned before over that track. If you view a picture of
a track taken with a Magnetic Force Microscope, you'll see shadows of
the previous positioning on the sides of the track. You can easily see 2
(left and right) additional tracks on the side, often more than one.
Take 3 past positions to the right and 3 to the left, plus the center
track, you end up with 7 shadowy tracks for one track. Guess how the NSA
got the number 7 :)

Cheers,
Frank

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