There are two primary procedures allowed by DoD policy for clearing and purging AIS memory and secondary storage media that have processed sensitive information: overwriting and degaussing.  Other procedures are media specific and this section details them where appropriate. The need for destruction arises when the media reaches the end of its useful life.
Overwriting is a process whereby unclassified data are written to storage locations that previously held sensitive data. To satisfy the DoD clearing requirement, it is sufficient to write any character to all data locations in question. To purge the AIS storage media, the DoD requires overwriting with a pattern, then its complement, and finally with another pattern; e.g., overwrite first with 0011 0101, followed by 11001010, then 1001 0111. The number of times an overwrite must be accomplished depends on the storage media, sometimes on its sensitivity, and sometimes on differing DoD component requirements. In any case, a purge is not complete until a final overwrite is made using unclassified data.
Degaussing is a process whereby the magnetic media is erased, i.e., returned to its initial virgin state. To satisfy the DoD requirement on degaussing a classified magnetic tape, the degausser must have met DoD testing requirements as discussed in Section 3, "Degaussers."
It is good practice to purge media before submitting it for destruction. Media may generally be destroyed by one of the following methods. (Although approved methods, options d and e use acid, which is dangerous and excessive, to remove recording surfaces. Options a, b, and c are recommended over d and e.)
a. Destruction at an approved metal destruction facility, i.e., smelting,
disintegration, or pulverization.
c. Application of an abrasive substance (emery wheel or disk sander) to a
magnetic disk or drum recording surface. Make certain that the entire
recording surface is completely removed before disposal. Also,
ensure proper protection from inhaling the abraded dust.
d. Application of concentrated hydriodic acid (55% to 58% solution) to a
gamma ferric oxide disk surface. Acid solutions should be used in a
well-ventilated area only by qualified personnel.
e. Application of acid activator Dubais Race A (8010 181 7171) and stripper
Dubais Race B (8010 181 7170) to a magnetic drum recording surface.
Technical acetone (6810 184 4796) should then be applied to remove
residue from the drum surface. The above should be done in a well-
ventilated area, and personnel must wear eye protection. Extreme
caution must be observed when handling acid solutions. This
procedure should be done only by qualified and approved personnel.
For additional information on destruction techniques and emergency destruction, see Institute for Defense Analyses (IDA) Report R-321, Emergency Destruction of Information Storing Media. 
DoD 5200.28-M provides accepted DoD procedures to clear, purge, declassify, and destroy storage media. This section, "Standards," is a reflection of those procedures but does not provide the entire procedure (e.g., use three overwrites to purge disks). This is because these standards are evolving and this document, A Guide to Understanding Data Remanence in Automated Information Systems, is not to be construed as replacing policy.
Although overwriting can be used for clearing this media, the method is time consuming and generally never used. Also, inter-record gaps may preclude proper clearing. A better method for clearing Type 1,11, and Ill tapes is degaussing with a Type l or Type II degausser. This procedure is considered acceptable for clearing, but not purging, all types of tapes.
Degaussing with an appropriate degausser is the only method the DoD accepts for purging this media. Specifically, a Type I degausser can purge only Type tapes, and Type II degaussers can purge Types l and Il tapes. No degausser presently exists that is capable of purging Type III tapes in accordance with NSA/CSS Specification L1 4-4-A.
The DoD has approved both overwriting and degaussing as methods to clear or purge this media. See Section 4, "Risk Considerations," and DoD 5200.28-M for additional information. Degaussed disks will generally require restoration of factory installed timing tracks. Type I degaussers and approved hand-held magnets can purge this media up to a coercivity level of 1100 oersteds. If hand-held magnets are used, then the magnet must be placed in almost direct contact with the disk, separated by only a tissue to prevent scratching the disk. Sometimes it is possible to insert the magnet between the platters without disassembling them. As a practical matter, if the drive must be disassembled, it is usually easier to destroy the platters than to degauss and then reinstall them.
Recently completed research has indicated that degaussing is an effective method to purge rigid disk media. Large cavity degaussing equipment can be used to erase the data from sealed disk packs and Winchester style hard disk drives while the platters remain in the drive. Care must be exercised to ensure that the disk drive is not encasqd in a material that conducts a magnetic field. Research has shown that aluminum housings on Winchester disk drives attenuate the degaussing field by only about 2 db. Operational guidance is now being developed for the DoD.
The DoD has approved both overwriting and degaussing as methods to clear or purge this media. See Section 4, "Risk Considerations," and DoD 5200.28-M for additional information. Type l degaussers and approved hand-held magnets can purge this media, with the latter being the only practical alternative.
The DoD has approved overwriting for clearing, but not purging, magnetic floppy disks. Degaussing is the preferred method. The technology of magnetic cards is old and not generally used. Degaussing with Type I degaussers or approved hand- held magnets is the only DoD accepted method of purging floppy disks and cards, regardless of their coercivity. See DoD 5200.28-M for additional information.
The DoD has approved both overwriting and degaussing as methods to clear or purge magnetic core memory. Type l degaussers and hand-held magnets can purge this media. See DoD 5200.28-M for additional information.
There are restrictions on overwriting magnetic plated wire memory based on the amount of time that information was resident in the same memory location. See DoD 5200.28-M for additional information.
The DoD has approved both overwriting and degaussing as methods to clear or purge thin film memory. Type l degaussers and approved hand-held magnets can purge this media.
The DoD has approved both overwriting and degaussing as methods to clear or purge magnetic bubble memory. An alternative procedure for magnetic bubble memory modules that have been designed with a built-in bias voltage control is to adjust (i.e., raise) the bias voltage to a level that would cause ttr;e collapse of all the magnetic bubbles. On some bubble devices a chip erase is invoked by pulsing the z-coil. If the memory was designed with a bias control, information will be available from the vendor on the correct bias voltage level to apply to cause the collapse of all the magnetic bubbles. Type l degaussers and approved hand-held magnets can purge this media. Degaussed bubble memory will generally require reinitialization with programs available from the manufacturer. Bubble memory has not been shown to exhibit any magnetic remanence after application of any of these purging methods.
The DoD has approved both overwriting and removal of power as methods to clear or purge RAM. See DoD 5200.28-M for additional information.
Because data is permanently stored in ROM, clearing and purging this media has no relevance. See DoD 5200.28-M for additional information.
The DoD has approved the use of ultraviolet light to clear or purge UVPROM. See DoD 5200.28-M for additional information.
The DoD has approved different forms of overwriting (e.g., single-step chip erase, individual overwriting, etc.) as methods to clear or purge EEPROM. See DoD 5200.28-M for additional information.