The timing with external hard drives is such that you must use the cable supplied with them and you must plug them directly into the computer. If you have a desktop with free expansion slots, you can add a card to increase the no. of USB ports, or you can plug things like printers into hubs, but the data transfer between the computer and the external drive has been optimized so that the timings are too critical for any other kind of connections. It's not just power, it's the data transfer timings as well.
Can you plug an unpowered USB hub into one of those usb to ac adapters then transfer data between things on the ports on it? Basically, I need to get extra power to my external hard drive that my Raspberry Pi can't supply, but still want to transfer data between the two. That would be cheaper than a powered usb hub. Just wondering if someone has tried it and if it works.