Functioning principles of Information wiping methods
Physically, the complete wiping of information from a hard disk involves the switching of every elementary magnetic area of the recording material as many times as possible by writing specially selected sequences of logical 1′s and 0′s (also known as samples).
Using logical data encoding methods in current hard disks, you can select samples of symbol (or elementary data bit) sequences to be written to sectors in order to repeatedly and effectively wipe confidential information.
Methods offered by national standards provide (single or triple) recording of random symbols to disk sectors that are straightforward and arbitrary decisions, in general, but still acceptable in simple situations. The most effective information-wiping method is based on deep analysis of subtle features of recording data to all types of hard disks. This knowledge speaks of the necessity of complex multipass methods to guarantee information wiping.
The detailed theory of guaranteed information wiping is described in an article by Peter Gutmann. Please see:
Secure Deletion of Data from Magnetic and Solid-State Memory.