"Ive run 3 external hds off a Belkin wall-powered hub and gotten the same speed as a direct connection, however my hard drives were all wall-powered as well. As a rule of thumb, it's definitely better to get powered USB hubs for any hard disk usage or heavy data access (bigger thumb drives, etc), while self/computer powered hubs are usually for mouse/keyboard and generally device inputs. There is no downside as far as I am aware of using a wall-powered hub, so if you have the extra cash, its always a good investment to go with external power. I personally use an old model of Belkin's hubs, and its served me well for almost 3 years now.
tl;dr: Drawing usb power is bad for lots of data transfer, grab an externally/wall powered hub and you should be fine. Try and get the hard drives themselves powered too -- the less stress on the hub, the better."
Each device reports to the hub its power requirements, and won't get initialized if it asks for more power than is available. Also, each standard USB 2.0 port can be asked to provide up to 500mA. If you have a self-powered hub, then the hub plus all devices attached must draw less than 500mA. A powered hub would be able to provide additional current to each connected device. Recommended: a powered hub, especially if you want to connect a self-powered hard drive.