I have a Nano that I would like to power via USB with a MP3/MP4 power supply, a tiny wall wart with a USB connector on it. I tried conecting it to the Nano. The Nano did power up, but the TX and RX LEDs were also on continuously. No permanent damage seems have occurred, but why were the LEDs lit? Is this bad for the Arduino?
I took the power supply apart and noticed that it connects USB pin 3 to ground, and has no connection on pin 2. Would it help to disconnect pin 3 from ground?