Seagate BlackArmor NAS 110, Руководство пользователя

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Appendix C. 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.
C.1 Information wiping methods’ functioning principles
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:
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html .