Other Methods of Erasing Magnetic Media
The previous section has concentrated on erasure methods which require no specialised equipment to perform the erasure. Alternative means of erasing media which do require specialised equipment are degaussing (a process in which the recording media is returned to its initial state) and physical destruction. Degaussing is a reasonably effective means of purging data from magnetic disk media, and will even work through most drive cases (research has shown that the aluminium housings of most disk drives attenuate the degaussing field by only about 2 dB [16]).
The switching of a single-domain magnetic particle from one magnetization direction to another requires the overcoming of an energy barrier, with an external magnetic field helping to lower this barrier. The switching depends not only on the magnitude of the external field, but also on the length of time for which it is applied. For typical disk drive media, the short-term field needed to flip enough of the magnetic domains to be useful in recording a signal is about 1/3 higher than the coercivity of the media (the exact figure varies with different media types) [17].
However, to effectively erase a medium to the extent that recovery of data from it becomes uneconomical requires a magnetic force of about five times the coercivity of the medium [18], although even small external magnetic fields are sufficient to upset the normal operation of a hard disk (typically a few gauss at DC, dropping to a few milligauss at 1 MHz). Coercivity (measured in Oersteds, Oe) is a property of magnetic material and is defined as the amount of magnetic field necessary to reduce the magnetic induction in the material to zero - the higher the coercivity, the harder it is to erase data from a medium. Typical figures for various types of magnetic media are given below:
Typical Media Coercivity Figures Medium Coercivity
5.25" 360K floppy disk 300 Oe
5.25" 1.2M floppy disk 675 Oe
3.5" 720K floppy disk 300 Oe
3.5" 1.44M floppy disk 700 Oe
3.5" 2.88M floppy disk 750 Oe
3.5" 21M floptical disk 750 Oe
Older (1980's) hard disks 900-1400 Oe
Newer (1990's) hard disks 1400-2200 Oe
1/2" magnetic tape 300 Oe
1/4" QIC tape 550 Oe
8 mm metallic particle tape 1500 Oe
DAT metallic particle tape 1500 Oe
US Government guidelines class tapes of 350 Oe coercivity or less as low-energy or Class I tapes and tapes of 350-750 Oe coercivity as high-energy or Class II tapes. Degaussers are available for both types of tapes. Tapes of over 750 Oe coercivity are referred to as Class III, with no known degaussers capable of fully erasing them being known [19], since even the most powerful commercial AC degausser cannot generate the recommended 7,500 Oe needed for full erasure of a typical DAT tape currently used for data backups.
Degaussing of disk media is somewhat more difficult - even older hard disks generally have a coercivity equivalent to Class III tapes, making them fairly difficult to erase at the outset. Since manufacturers rate their degaussers in peak gauss and measure the field at a certain orientation which may not be correct for the type of medium being erased, and since degaussers tend to be rated by whether they erase sufficiently for clean rerecording rather than whether they make the information impossible to recover, it may be necessary to resort to physical destruction of the media to completely sanitise it (in fact since degaussing destroys the sync bytes, ID fields, error correction information, and other paraphernalia needed to identify sectors on the media, thus rendering the drive unusable, it makes the degaussing process mostly equivalent to physical destruction). In addition, like physical destruction, it requires highly specialised equipment which is expensive and difficult to obtain (one example of an adequate degausser was the 2.5 MW Navy research magnet used by a former Pentagon site manager to degauss a 14" hard drive for 1« minutes. It bent the platters on the drive and probably succeeded in erasing it beyond the capabilities of any data recovery attempts [20]).