Bad Durandal? - USB connection connects and disconnects

Have a new Durandal that happily uploads firmware in Mission Planner but will not let me reconnect later in MP. Unit connects to PC, (confirmed with new port showing up in Device Manager and MP), and disconnects ~5 seconds later. If you hit CONNECT in MP during that brief period, you get an error saying that the COM port chosen “does not exist”. During that brief period of connection, all I/O lights are on and the PWR light on the FMU side is flashing. Then it disconnects and the FMU side goes dark. Have tried multiple PCs, different cables, reloaded Misson Planner, etc with same result. My other Durandal connects and works flawlessly. Any ideas besides a bad Durandal?