Arduino Nano v3.0 USB connectivity after external power

I have a strange problem on all arduino nano v3.0 clones i have (funduino)

When i plug the mini usb jack the arduino nano is discovered by my windows 8.1 operating system
But when i first connect an external power on the arduino (e.g using the VIN pin) and then i try to connect the usb, the usb connection does not work. The arduino leds are not blinking...

Any idea why?