Win 2003: Permanent drive letter 4 removable drive

I have set up a backup system on Windows 2003 Server using 2 hot-swap removable drive drawers with hard drives in them. I have a pair of backup drives for each day of the week, with one drive imaging the OS partition and the other drive imaging the data partition. The system is working beautifully except for one problem.

I cannot work out how to get the OS to PERMANENTLY assign the drive letters S for the system backup drive and T for the data backup drive. (Also remember that the actual hard drives being used change daily, as that might be an extra complication.) The problem arises as soon as I change backup drives for the day. They are immediately recognized and assigned drive letters F & G rather than the drive letters S & T that I would like to assign them and which are set up in the backup software.

I do not wish to use F & G as there are times when a USB drive or other device will be mounted and that will throw the whole system out of whack once F and/or G are assigned to the USB drive(s). I need to be able to hot-swap and know that the drives will be assigned S & T so that the late-night backup will go through without problems.

I would appreciate any help and advice with this problem.