It's a long time since I used an Arduino, so I can't remember what I did.
Did you cut both +5v and GND wires in your USB cable?
If so, that's probably the cause. I think I only cut the +5v cable.
Without the GND wire, I'm not sure the USB signals will be recognised, even though they are differential.
After my success with the cable I progressed onto modifying my Arduino's and actually cut the +5v track leading out from the USB connector. Not for the faint hearted but it worked for me on the boards I modified.