Running the blink sketch on a Uno via USB worked fine on USB power but the sketch would not start when running on external power (USB disconnected).
- Connecting Rx to GND through a 10k resistor as recommended in Troubleshooting did not work
- Connecting RX to Tx as recommended in Troubleshooting did not work
- Connecting Reset to GND via a 1 microfarad capacitor and a 10K resistor did workThis is very similar to the experience reported by Mike Hains in Works on USB, does not start on External Power, blink sketch (SOLVED ?) - Installation & Troubleshooting - Arduino Forum