Increasing USB 5V current the right way
You seem to have the right idea about how this should work. If you hook up two separate power supplies set to the same voltage, there will always be a small difference that will cause current to flow from one source to the other. Inevitably, the "stronger" supply will tend to drive the weaker supply higher or lower to match it. Usually not a good thing. Connecting two power sources in parallel is not impossible, however. For example, you can connect two parallel battery packs to your circuit by just using diodes in series with each one. The pack with higher voltage will initially take most of the load, but as it drops in voltage from use, the other pack will start to take over. Eventually the load will draw from both packs equally.

If your load will draw a large current (much greater than 500mA), #2 on your list is probably the best idea. And yes, you would want to tie the ground of the USB port to the ground of your power supply.

Incidentally, you've got a hefty 5V power supply right there in your computer (assuming you have a tower PC... not applicable to a laptop). The red wires on any unused peripheral power plug are 5V, yellow is +12V, and black is ground. The other colors are negative voltages, which I assume aren't useful to you right now.


If your load will only draw a little more than 500mA, but a bit less than 1A, you could also just use two USB ports together in parallel. This is common in external USB disc drivers. They come with two USB plugs on them. One has power and data, the other just power.