I use UNO with MacBook(SnowLeopard) and Arduino 022.
When connect USB to UNO board (no external power supply), then orange LED ("L" silk) continues to flash, and the sketch program does not start.
I can write a sketch in this state, and the sketch works after writing. However, when unplug and re-connect the USB, then LED continues to blink again, and the sketch doesn't start.
(I think, AVR's EXTRF bit in MCUSR does not clear, somehow.)
This situation occurs at external power supply, too.
What should I do?