Retaining drive letters with esata

I have a customer running a Dell PowerEdge 1900. we've recently installed an eSata PCI-X controller, and have it connected to an eSata drive docking station. When the customer puts in one of their 2 TB drives to backup their data from the Server, the drive letter shows up as drive letter G. However, the problem is when the backup is done, and they remove the drive, and put in a newly formatted drive, it comes up with a different drive letter. We're currently using the NTbackup from within Windows 2003 Server. Is there a way to retain drive letters on eSata without having to go into Disk Management and manually changing the drive letter every day?