I am having an issue with external loads causing the computer I have my Arduino connected to to momentarily drop out, which causes the Arduino to lose USB power momentarily and the port drops out and won't reconnect unless I physically remove and reconnect the USB cable. Since I need the USB connected in order to send and receive serial data to/from the Arduino, I was wondering if I could physically open the USB cable, cut the 5V line, and supply it via and external regulated 5V supply. I wouldn't cut the ground wire, but would tie it to the external 5V supply. I have read conflicting information on this (something about handshaking) but was wondering if that applied to Arduino's specifically.
I solved this problem by using a laptop instead. It is immune to these power fluctuations
If you are drawing more than 500mA from your PC then that is why it is happening.
I suggest, rather than butchering a cable, you get yourself a "wallwart" (Mobile phone charger) and use that to power the board. You really should work out how much current you are pulling before deciding on what PSU to use.
A problem with wall warts is earthing, you need to make sure that your PC and wall wart are connected to the same earth connection, so that they share a common ground.
If the current being drawn us only for a very short time then it might be possible to provide additional power using a capacitor in the power line. But plugging a charger in is much easier.