The embodiments described herein generally use a challenge to protect a removable mobile flash memory storage device, where the challenge may be in the form of a “Completely Automated Public Turing Test to Tell Computers and Humans Apart” (“CAPTCHA”). In one embodiment, a method is provided in which a removable mobile flash memory storage device receives a command from a host device, generates a CAPTCHA challenge, provides the CAPTCHA challenge to the host device, receives a response to the CAPTCHA challenge from the host device, determines if the response satisfies the CAPTCHA challenge, and performs the command only if the response satisfies the CAPTCHA challenge. In another embodiment, a removable mobile flash memory storage device is provided for performing these acts.