Hello. I have the same problem. I received my r3 board from ebay few month ago. I had no time to test it. Now I bought the USB cable. It powers from USB. Blinks all the time but no of 4 windows or linux PC see it. Tried another cable- doesn't help. I have Uno v3 and Nano boards.
Can somebody point me to tutorial how to check/fix atmega16u2 usbserial?
I can't send it back as i lost seller contacts ;( don't want to send it to trash too.