How do devices that can be powered either from a wall wart or from USB work to avoid feeding power back to the USB host? Take a powered USB hub for example -- many will operate with just the USB input, but can also take a "wall wart" SMPS to provide higher current supplies. While super cheap ones just tie the two +5V lines together, I imagine there's a more proper way to do this. Obviously, 2 diodes would work, but then you have a diode voltage drop, which isn't really usable to power USB from USB.