"tty.usbserial" disappears when ext. power

Maybe a newbie question, but I haven't been able to find a solution for it so far in the forum.

When I connect my Diecimila to my Macbook Pro via USB and have the power jumber on the "USB" pins, my serial connection in the Arduino IDE shows up under "Tools|Serial Port" as "/dev/tty.usbserial-XXXXXX" (where XXXXXX denotes the specific name for the usb port).
When I move the power jumper to the "EXT" power pins and connect Vin to 12V and Gnd to 0V, and reconnect the USB plug, the tty.usbserial port is gone.

I would very much like to be able to monitor what's happening in the Arduino while I develop my sketch, so I need to be able to use the tty.usbserial port to connect with my Arduino.

Does anyone know a solution to this? Or why the usb.serial port doesn't show up?

Best Regards,

Jakob Bak