The M model radios have an additional set of processors that interface the front panel with the radio RF section and these processors need sufficient time to initialize and shut down to ensure the proper operation of the radio.  When using an "M" model radio, it is important that the radio is allowed to shut down completely before turning off the 13.8 VDC power supply.  Failure to do so may result in the radio not completely saving the operating state information which may prevent the radio from not starting up properly.