Eraser Forum

The main purpose of overwriting is to alter the magnetic polarity of each domain on the disk platter as much as possible so it will be extremely hard to determine their previous state.

If the data was written directly to the disk, files could simply be overwritten with patterns consisting only of ones or zeros. However, various run-length limited encoding algorithms are used in hard disks to prevent read/write head from losing its position and therefore, only limited amount of adjacent ones or zeros will be written to the disk. This is why different encoding schemes must be taken into account when selecting overwriting patterns.

In his paper Secure Deletion of Data from Magnetic and Solid-State Memory, Peter Gutmann has discussed the subject further. In chapter Erasure of Data stored on Magnetic Media he suggests a 35 pass overwriting method which should erase the data despite the drive encoding and this method is used as the default overwriting method for Eraser.

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Data on floppy disks can sometimes be recovered by forensic analysis even after the disks have been overwritten once with zeros (or random zeros and ones).[21] This is not the case with modern hard drives:

According to the 2006 NIST Special Publication 800-88 Section 2.3 (p. 6): "Basically the change in track density and the related changes in the storage medium have created a situation where the acts of clearing and purging the media have converged. That is, for ATA disk drives manufactured after 2001 (over 15 GB) clearing by overwriting the media once is adequate to protect the media from both keyboard and laboratory attack

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