|You probably need to connect the USB external drive to a different USB port. |
The following has some info that applies to PCs and Windows, but the same things apply to using a USB device or a USB external hard drive on any computer.
USB devices may not work correctly when they're connected to certain USB ports. Instructions for how to re-load your USB stack in 2000 or XP, if you need to - similar applies to Vista and Windows 7.
In addition, all external drives connected by USB require that the port (or ports) they connect to can actually supply 500ma of current.
USB external hard drive requirements.
See Response 9: