I have a pair of Duemilanovas. When I attempt to upload a program (I'm running Arduino 18 on a Mac with OSX 10.4.11) there's no problem with one of the boards...I can find the correct USB serial port in "Serial Port" list in the Arduino IDE.
When I connect the other board however, the relevant serial port no longer appears in the list in the IDE.
My first thought was that the board is somehow defective, but I seem to recall it working a couple of days ago, and I didn't do anything to fry the Arduino as far as I can tell.
Is there anything I can try to get this board to communicate with my Mac again