Clearing (media sanitization)

Clearing is the least intensive of the three sanitisation methods in NIST SP 800-88, Guidelines for Media Sanitization, whose current revision is Rev. 2 (2025). It applies logical techniques to all user-addressable storage locations, typically overwriting them with new values through the device’s standard read and write commands, or using a factory reset where overwriting is not supported. The aim is protection against simple, non-invasive recovery, such as undelete tools.

The limit sits in the words “user-addressable”. Clearing does not reach areas the interface does not expose, such as remapped bad sectors or the spare blocks that wear levelling uses on flash storage. Overwriting is the usual clearing technique. Purging, which includes techniques such as degaussing and cryptographic erase, is designed to defeat state-of-the-art laboratory recovery, and physical destruction does the same while also leaving the media unusable. The decision flow in SP 800-88 Rev. 2 points to clearing only for media that is being reused, stays under organisational control and holds low or moderate impact data, and the guideline prefers purging wherever it is possible.

Exam relevance: a scenario is likely to turn on where the media goes next. Reuse inside the organisation at a similar sensitivity may justify clearing, while media leaving organisational control, or holding highly sensitive data, tends to call for purging or destruction. Candidates are expected to remember that clearing an SSD by overwriting can leave recoverable data behind.