As I understand it, when you power the Arduino from an external supply, the serial functions do not work. The Serial pins only work when the Arduino is powered by USB.
I came up with a possible solution, was wondering if anyone has already tried it though:
I'm thinking of cutting a USB cord and abandoning the power supplied by the computer. This is on 2 of the 4 pins. Leave them disconnected. Next, provide the +5v wires on the arduino side of the cut plug with an external supply of +5v. Tie GND from the PC side of the cut USB cable and the external 5v supply.
This should, in theory, provide the arduino with 5V on the USB cord and enable serial routines, while having the data lines of the USB still connected to the PC. This way, (I hope) when the computer is off the arduino can run from the external power supply. When the computer is on, that will not change but serial routines should be available on the arduino.
Has anyone tried it this way before? In theory it seems it would work correctly, but was wondering if anyone has attempted this yet.