Hi Chris
Try restarting your PC while External HD is plugged in, this way it might read/load it properly.
Try unplugging other devices from USB ports and then plug in your External HD only. Usually keyboard and mouse you can leave plugged in. But many other devices such as WiFi Netgear stick or similar can cause it not to work.
If that doesnt work, try plugging it on different computer and see if it works.
If none of it work read this:
http://www.tech-faq.com/io-device-error.html
cheers