Last technical review: June 29, 2026

“Read-only” sounds like a simple property: a storage device either allows changes or it does not. In practice, the phrase describes a rule enforced somewhere along the path between an application and the storage media.

That distinction becomes important when a Multipurpose USB Device (MUD) travels between Windows, macOS, Linux, ChromeOS, and computers that may not be fully trusted. A computer may report that the MUD is read-only while the device itself remains physically capable of accepting writes. Connect it to a different host—or start the same computer under a different operating system—and the rule may no longer apply.