Arduino power sources Is it safe to connect a USB input to an Arduino (Nano) at the same time as an external regulated 5V supply to its 5V pin is connected, assuming the supplies have common grounds ? Will (I hope) the external 5V be used so as to not draw too much current out of the USB supply ?
From the Arduino Nano at Arduino.CC:


  The Arduino Nano can be powered via the Mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source.


Or, take if from the schematic: