There are a few things that could cause this kind of problem. Here's a short list out of the dozen or so possibilities:
If it's not one of these three then it could be caused by corrupted system files, a damaged registry, or a host of other operating system problems. Frequently these types of problems can be repaired, but without extensive knowledge of the inner workings that may require professional assistance.
- INFCACHE.1 Corruption. It's possible that the INFCACHE.1 file located in C:\Windows\Inf could've become corrupt or might not have been built properly. This file is a cache of all driver data that's built whenever a new driver is installed. If the file is deleted, it will rebuilt itself on the next device detection or reboot. Sometimes simply deleting this file and rebooting is enough to get things going.
- USB Power Problems. Some USB devices, particularly external hard drives, are very sensitive to power input. They may require more power than other devices, like USB mice. If you've got the hard drive attached to a USB hub, make sure it's self-powered or try attaching it directly to the computer itself. Try to use the original cable that came with it as some cables may not properly carry the 500mW required by USB hard drives.
- BIOS/Firmware issues. Some systems may require a BIOS or firmware update to get certain USB devices working,a s slidman mentioned. Usually this can be found through the manufacturer and an update may not be available for every system.