When the Iranians took over the American Embassy in Teheran they, acquired access to the machines there. Subsequently there was talk of computer systems that were guaranteed to be volatile except for ciphered disks. There would be an unciphered boot block on disk that did not have the key to the rest of the disk but did have code to read and decipher the rest of the operating system. That key, however, would be in a safe place such as Washington DC. The system could not be booted until the key was available, presumably thru secure communications. If you trusted the operating system to only use the key for reading and writing the disk but not otherwise then pulling the plug made the all data in the computer inaccessible baring action from Washington.