The maximum length of a single USB cable is 16.4 feet (to be smooth as a high-speed, very chatty transport they need to have fast acknowledgements of messages on the bus and propagation delays rapidely becomes an issue). Beyong that, you need to insert hubs (repeaters), which are much more than just cable extensions.

As for power, the bus does not provide THAT much power. Mechanical devices such as harddrives would either drain it all or run out of juice anyway.

See Mojo's signature