Copyright © Acronis, Inc., 2000-2010 193
24.3 Hard Disk Wiping methods
Information removed from a hard disk drive by non-secure means (for example, by simple Windows
delete) can easily be recovered. Utilizing specialized equipment, it is possible to recover even
repeatedly overwritten information. Therefore, guaranteed data wiping is more important now than
ever before.
The guaranteed wiping of information from magnetic media (e.g. a hard disk drive) means it is
impossible to recover data by even a qualified specialist with the help of all known tools and recovery
methods.
This problem can be explained in the following way: Data is stored on a hard disk as a binary
sequence of 1 and 0 (ones and zeros), represented by differently magnetized parts of a disk.
Generally speaking, a 1 written to a hard disk is read as 1 by its controller, and 0 is read as 0.
However, if you write 1 over 0, the result is conditionally 0.95 and vice versa – if 1 is written over 1
the result is 1.05. These differences are irrelevant for the controller. However, using special
equipment, one can easily read the «underlying» sequence of 1's and 0's.
It only requires specialized software and inexpensive hardware to read data "deleted" this way by
analyzing magnetization of hard disk sectors, residual magnetization of track sides and/or by using
current magnetic microscopes.
Writing to magnetic media leads to subtle effects summarized as follows: every track of a disk stores
an image of every record ever written to it, but the effect of such records (magnetic layer) becomes
more subtle as time passes.
24.3.1 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.
24.3.2 Information wiping methods used by Acronis
The table below briefly describes information wiping methods used by Acronis. Each description
features the number of hard disk sector passes along with the number(s) written to each sector byte.
