[MAGZine]

AFAIK, the FDR and CVR can both be disabled by crew to some degree. I think they can be disconnected from the aircraft's power system (by fuse/breaker), but they also have some amount of battery backup.

Part of Federal Express Flight 705's whole debacle was that the hijacker tried to pull the breaker on the CVR.

And the investigation into SilkAir 185 was hampered because the recorders had their power pulled as a apart of load shedding while trying to resolve issues onboard the aircraft.