i realise that some of the following does not pertain to your situation exactly, but you'd do well to check it, it's all USB troubleshooting...
excerpted from another Apple artnum: http://til.info.apple.com/techinfo.nsf/artnum/n43048#c
For USB devices:
If you're still having problems with one or more USB devices, do the following:
* Plug the device into another USB port on your computer or into the USB port on another device that is plugged into a power outlet (such as a self-powered hub).
* If the device is connected to a hub, try unplugging one or more devices from the hub.
* If the device is connected to a self-powered hub (a hub that has a power cord), make sure the cord is plugged into a working electrical outlet.
* Unplug all USB devices from your computer except the Apple USB keyboard and Apple USB mouse. Plug one device in at a time to determine which device is the problem. Once you determine which device is causing the problem:
* There may be something wrong with the cable connecting the device. If possible, borrow a cable that you know works with another similar device. Connect the cable to the device, then connect the device to your computer. Start up your computer. If the device works using the borrowed cable, then you need to replace the cable. Contactthe device manufacturer or an Apple-authorized service provider.
* Refer to the documentation that came with the device. Also see the information on connecting equipment in the online documentation that came with your computer.
* If you suspect a mouse problem, clean your Apple USB mouse as described in the online documentation that came with your computer. Then, plug the mouse into an open USB port on your computer. If the mouse works, then you may have a problem with the keyboard. If the mouse doesn't work, borrow an Apple USB mouse from another computer and connect it to yours. If that solves the problem, you may need to repair your mouse. Consult an Apple-authorized service provider.
* If you suspect a keyboard problem, borrow an Apple USB keyboard from another computer and connect it to yours. If that solves the problem, you may need to repair your keyboard. Consult an Apple-authorized service provider.
and if you go here: http://til.info.apple.com/techinfo.n...+search+simple
, and type in 'USB keyboard' in the search field, you will find many more articles on the subject.