Arduino Mega 2560 - sudden unknown USB device

Development Tools IDE 1.x
3

I have had similar problems which can show as:

  1. Tools / Serial Port grayed out
  2. Serial Portx already in use.
  3. Hangs on upload
  4. avrdude not in sync.
    These are caused by not downloading the right drivers. The Arduino IDE automatically downloads drivers, but does not always get the right ones. This is especially a problem if you are working with two different boards, e.g. Mega and Nano. Follow your board documentation to identify the right driver, and manually download it.