Source : Free On-Line Dictionary of Computing

locked and loaded
     
        [Military slang for an M-16 rifle with magazine inserted and
        prepared for firing] Said of a removable disk volume properly
        prepared for use - that is, locked into the drive and with
        the heads loaded.  Ironically, because their heads are
        "loaded" whenever the power is up, this description is never
        used of {Winchester} drives (which are named after a rifle).
     
        [{Jargon File}]