Technical Overview
One standard way to recover data that has been overwritten on a hard drive is to capture and process the analog signal obtained from the drive's read/write head prior to this analog signal being digitized. This analog signal will be close to an ideal digital signal, but the differences will reveal important information. By calculating the ideal digital signal and then subtracting it from the actual analog signal, it is possible to amplify the signal remaining after subtraction and use it to determine what had previously been written on the disk.
For example:
Analog signal: +11.1 -8.9 +9.1 -11.1 +10.9 -9.1 Ideal Digital signal: +10.0 -10.0 +10.0 -10.0 +10.0 -10.0 Difference: +1.1 +1.1 -0.9 -1.1 +0.9 +0.9 Previous signal: +11 +11 -9 -11 +9 +9This can then be done again to see the previous data written:
Recovered signal: +11 +11 -9 -11 +9 +9 Ideal Digital signal: +10.0 +10.0 -10.0 -10.0 +10.0 +10.0 Difference: +1 +1 +1 -1 -1 -1 Previous signal: +10 +10 -10 -10 +10 +10However, even when overwriting the disk repeatedly with random data it is theoretically possible to recover the previous signal. The permittivity of a medium changes with the frequency of the magnetic field. This means that a lower frequency field will penetrate deeper into the magnetic material on the drive than a high frequency one. So a low frequency signal will, in theory still be detectable even after it has been overwritten hundreds of times by a high frequency signal.
The patterns used are designed to apply alternating magnetic fields of various frequencies and various phases to the drive surface and thereby approximate degaussing the material below the surface of the drive.
Read more about this topic: Gutmann Method
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