Conflicts with USB and external power supply I want to power an Arduino Leonardo with an external power supply and simultanously communicate via USB. I have read some other threads on this topic, therefore, I know that if one supplies more than 7V over the power jack, the board will use the external power supply in this case.

My suspicion is rather concerning the ground connection. From the schematic, I can tell that the USB ground is connected via a tiny coil to the board ground, so with DC it is shorted. Wouldn't that cause problems if the computer's USB ground is at a different potential than the external power supply of the board, resulting in a voltage between the two grounds? What if this voltage is up to several Volts?

Thanks a lot!
Why do you suspect a potential difference? Where is th 7V coming from? If the 7V power supply is free floating and used for nothing else, there is no source for potential difference.

A typical wall wart will have a free floating output.