Hi! > Why 20 times? How is it possible to recover a file that has been overwritten once?
Because hard drives use magnetic imprints (I don't know the exact term for that), even once the magnetic information has been replaced by other information, the old one should still be present as a kind of ghost magnetic field. Obviously, such a ghost cannot be recovered with the tools provided with any OS. However, I guess that if a well organized institute or electrical engineer opens your hard drive and start analyzing the disks themselves with advanced machines, they could "see" this ghost and extract the old information out of it. It may sound like pure sci-fi, but I'm sure NSA has just the right tools for the task! ;-) When "shred" does 10 or 20 overwrites, I guess it alternates the bit patterns at each pass, such as to "scramble" the ghost information. Because one would have to actually "open" the hard drive and use advanced machinery to extract information that was overwritten *once*, it does not mean that doing a simple "delete" is sufficient, since in this case it is not overwritten at all, which allows for a simpler recovery with OS tools. By the way, anybody knows what are the Linux tools for that? Something similar to DOS "undelete"? Mathieu.
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