Wear leveling
A flash controller technique that spreads writes across memory cells by remapping them, which can leave old copies of data where host-based overwriting may not reach.
Wear leveling (wear levelling in UK spelling) is a technique used by the controllers of flash storage, such as solid-state drives and USB drives, to spread writes evenly across the memory cells. Flash cells survive a limited number of program and erase cycles, so the controller commonly writes each update to a different physical location and updates its map. Drives also keep spare capacity, known as over-provisioning, that the host operating system cannot address.
For sanitisation this is the central problem. Because a logical address is not tied to a fixed physical cell, overwriting a file, or even a whole drive, from the host can leave older copies of data in remapped or spare cells, a form of data remanence the host cannot see. Degaussing does not help, because flash storage is not magnetic. NIST SP 800-88 Rev. 2 names wear levelling and spare cells as the reason host overwriting cannot reach all previous data on flash, and points instead to purge techniques that act inside the device, such as the drive’s own sanitize commands for block erase and cryptographic erase, or to physical destruction. A self-encrypting drive makes cryptographic erase straightforward.
Exam relevance: a scenario is likely to describe an organisation overwriting or degaussing SSDs before resale or reuse. Candidates are expected to recognise that wear leveling makes host-based overwriting unreliable on flash media, and to prefer cryptographic erase, the drive’s own sanitise command, or destruction.