Seagate USB HDD not showing up

It works on my computer (XP), but when put on my brother's computer (with vista) it does not get assigned a letter. I've read every posting about this I can find on google, even tried a couple of programs designed to assign letters to USB devices and neither worked. It shows up in device manager fine, but when I go to disk manager it doesn't allow me to change the letter.

It doesn't give me the option at all. It's partitioned into two partitions and when I click on them it gives me the option but it's grayed out so I can't click it. Grrr!

The computer has 2 internal HDD, 3 DVD drives, and 4 removable HD in it. I changed the 3 DVD drives to X, Y, and Z, one of the internal HDD from D to W, and the 4 removable ones to P, Q, R, and S. Then I reinstalled the external HDD. Still nothing.

There are no network drives at all. And I assume it's the admin login because it's the only user set up.

Anyone have ANY ideas???

First, See How to Cleanup and Remove old USB Storage Drivers

Thanks for getting back to me. I followed those instructions and still it's not working. What's next?

Could you provide a screen shot of what you're seeing in Windows Disk Management?

Well, I don't really know how to do that, unfortunately. lol

It shows two partitions and when I click on either partition all the options are grayed out. When I click on the left side that says "Disc 6" Basic all the options are grayed out except "make dynamic".

It should also be noted that it doesn't say anything under how it's formatted. The other HDD say "NTFS".