Also, it can occur that the right device in the BIOS menu to set is not always obvious- for example, on my machine there are about 4 different options for USB connected devices, and it's not obvious which one to choose. Also, there have apparently been cases where it shows up as an additional HDD rather than a USB device at all. Obviously I don't know your circumstance in this area, but perhaps you think you've set the right option but haven't?
To make sure you've done all the right steps, try a look through this guide:
http://fuzzymunchkin.dyndns.org/tdot/?page_id=632