Powering the arduino with a 5V power supply

MeatPlatter:
Do you know what happens if I supply voltage to the 5V pin and also have the USB connected to my computer?

You can't do it. The different power supplies will fight each other.

(Never connect the outputs of two power supplies together...)

That's why the USB cable method is doubleplusgood. You have to remove power to be able to connect your PC.