Editing Degaussing (section)

==Magnetic data storage media==
Data is stored in the [[magnetic media]], such as [[hard drive]]s, [[floppy disk]]s, and [[magnetic tape]], by making very small areas called [[magnetic domain]]s change their magnetic alignment to be in the direction of an applied magnetic field. This phenomenon occurs in much the same way a compass needle points in the direction of the Earth's magnetic field. Degaussing, commonly called erasure, leaves the domains in random patterns with no preference to orientation, thereby rendering previous data unrecoverable. There are some domains whose magnetic alignment is not randomized after degaussing. The information these domains represent is commonly called magnetic [[remanence]] or [[remanent magnetization]]. Proper degaussing will ensure there is insufficient magnetic remanence to reconstruct the data.<ref>{{cite web |title=A Guide to Understanding Data Remanence in Automated Information Systems |url=http://www.cerberussystems.com/INFOSEC/stds/ncsctg25.htm |website=www.cerberussystems.com |archive-url=https://web.archive.org/web/20160304103802/http://www.cerberussystems.com/INFOSEC/stds/ncsctg25.htm |archive-date=2016-03-04 |url-status=dead }}</ref>

Erasure via degaussing may be accomplished in two ways: in [[Alternating current|AC]] erasure, the medium is degaussed by applying an alternating field that is reduced in amplitude over time from an initial high value (i.e., AC powered); in [[Direct current|DC]] erasure, the medium is saturated by applying a unidirectional field (i.e., DC powered or by employing a [[Permanent magnet#Types of permanent magnets|permanent magnet]]). A ''degausser'' is a device that can generate a magnetic field for degaussing magnetic storage media.<ref>{{cite book |author=((National Computer Security Center)) |chapter=3. Degaussers |chapter-url=https://books.google.com/books?id=8FfatWIK5lQC&pg=PA11 |title=A Guide to Understanding Data Remanence in Automated Information Systems |date=1995 |isbn=9780788122279 |url=https://books.google.com/books?id=8FfatWIK5lQC |orig-date=September 1991 |id=NCSC-TG-025 Library No. S-236,082 Version-2 |pages=11–|publisher=DIANE }}</ref> The magnetic field needed for degaussing magnetic data storage media is a powerful one that normal magnets cannot easily achieve and maintain.<ref>{{cite web |title=TOP 5 HARD DRIVE DESTRUCTION METHODS THAT WORK |url=https://datadestruction.com/top-5-hard-drive-destruction-methods-actually-work/ |website=Data Destruction |date=15 March 2020 |access-date=3 October 2021}}</ref><ref name=vs>{{harnvb|Verity Systems}}</ref>

=== Irreversible damage to some media types ===
Many forms of generic magnetic storage media can be reused after degaussing, including [[reel-to-reel audio tape recording|reel-to-reel audio tape]], [[VHS]] videocassettes, and [[floppy disk]]s. These older media types are simply a raw medium which are overwritten with fresh new patterns, created by fixed-alignment read/write heads.

For certain forms of computer data storage, however, such as modern [[hard disk drive]]s and some [[tape drive]]s, degaussing renders the magnetic media completely unusable and damages the storage system. This is due to the devices having an infinitely variable read/write head positioning mechanism which relies on special servo control data (e.g. [[Gray code#Position encoders|Gray Code]]<ref>{{cite book |title=Official Gazette of the United States Patent and Trademark Office: Patents |date=2001 |publisher=U.S. Department of Commerce, Patent and Trademark Office |page=1977 |url=https://www.google.co.uk/books/edition/Official_Gazette_of_the_United_States_Pa/9Qzw-6c9OeUC?hl=en&gbpv=1&dq=gray+code+magnetic&pg=PA1977&printsec=frontcover |language=en |chapter=US patent 6,201,652 B1}}</ref>) that is meant to be permanently recorded onto the magnetic media. This servo data is written onto the media a single time at the factory using special-purpose servo writing hardware.

The servo patterns are normally never overwritten by the device for any reason and are used to precisely position the read/write heads over data tracks on the media, to compensate for sudden jarring device movements, thermal expansion, or changes in orientation. Degaussing indiscriminately removes not only the stored data but also the servo control data, and without the servo data the device is no longer able to determine where data is to be read or written on the magnetic medium. The servo data must be rewritten to become usable again; with modern hard drives, this is generally not possible without manufacturer-specific and often model-specific service equipment.