I did this too, so that I could have two power supplies, one for the Pi and one for the HDD. Only the ground and data lines are common to both devices. Right now (as mentioned) both devices are working off a single 2A PSU.rpdom wrote:You don't need to damage any of the ports. Just a cable. Make a careful cut in the outer insulation, pull out a bit of the red wire, cut the wire and carefully isolate it with tape. Make sure it can't touch anything else, like the braided shield around the cable.
I had another idea, which I shall try if my current configuration stops working. I have seen power supplies for iPhones which have two USB outputs. One is rated at 2.1A, and the other is rated at 1A. The total output is 3.1A. I could buy one and try connecting the hard drive to the 2.1A output, and the Pi to the 1A output. If they are truly independent then a load on the 2.1A output should not cause the Pi to brown out.