I did a little research and found some instructions that might help you. It is possible that you may have a problem with the BIOS on your motherboard, but before going there try this simple little trick.

We want to cut power to the hard drive in question. Unplug the power cord for the external hard drive for approximately 25 minutes. This is important as 10-30 seconds is not usually long enough. If you have a laptop that has the same problem then you must also remove the battery on the laptop to ensure that no power is reaching it.

After the elapsed time, then plug the hard drive back in and re-connect the hard drive to the computer to see if things are now working.

I have also seen some other instructions that have been posted in a few different forums with success. Maybe this will help.

Do you by chance have some mapped network drives? If so, make sure you have not mapped any to the next drive letter that comes after your local drives. For example:

You have local drives: A:, C:, and D: which are a floppy disk, hard drive, and DVD drive. You then map a network drive to E:. If you next plug in a USB drive, a bug in Windows will cause it to decide that E: is what it should map the USB drive to, causing a collision. All kind of weird results can happen, but one of them is that you cannot see the contents of the drive.

Thus, the fix is to make sure you map your network drive higher up. For instance with network drives start with Z: and work your way backward in the alphabet that way all of your local drives will come first, and all of your network drives will come last.

There used to be a Microsoft support article Q297694 that discussed this.

I had the same problem with Windows 7. I think it's the same for Windows Vista. Right-click in the Royal Blue bar, and select New Simple Volume. Name the partition, and give it a letter, then give the settings you want. That should solve the problem for you.