I saw your post in August regarding mounting a USB disk on a laptop running winsowa 2003 server. I had exactly the same problem and the follinwg advice worked perfectly...

Windows Server has a tendency of attempting to assign a drive letter to the USB hard drive that is already in use. Open Disk Management and look at the current disks. You will probably see your USB Drive but there is not a letter assigned to it. Go to that device, right click on it, and then click on CHANGE DRIVE LETTERS AND PATH. You will then see a place to change the drive letter. Simply change it to a letter that is not being used already and then CLICK OK. You problems are now resolved. The operating System will assign the letter of your choosing and from then on.

If you have solved the problem, apologies for bothering you.

Regards,