I just looked at the last fixed harddrive letter (in this case CDEFG were all being used), plugged in a USB key and assigned it one more than the next available letter which in this case is the letter is I. (I set the key to hold a backup of the system state data since I didn't want it to just sit there doing nothing BTW.)
We then set set the backup software to look for the H drive. Because with drive I now 'permanently' occupied by the USB key, it leaves a gap in the sequence, with the next available letter being H. If you remove and then immediately add (as in swap) an external drive, it will always assign it the same letter (H) since it's the next available in sequence (i.e. CDEFG_I).
So why create a gap in the letter sequence? Here's why: Anything that gets plugged in after that will get letter J or higher.-40hz
Hm... Either there's a hole in your theory, or the server didn't read the same manuals that you did...

The used letters on that machine were:
C: - Physical
D: - CD-ROM
F: - Mapped Drive
S: - Physical
So from what you're saying it should pick T: next, yes? It didn't/doesn't *Shrug* I got tired of fiddling with it and Moved the CD-ROM to M: (because I was Mad) so the backups could use D: without tripping over the mapped F: Drive (always a good time).
Don't ya just love it when what's supposed to happen and what does happen aren't even close...

So as long as you're swapping H properly using the remove USB applet - and you immediately replace it with another drive - the drive letter shouldn't change. The backup drive will always get the letter that falls into the gap. And any additional USB drives will grab letter J or higher so it won't affect your designated backup drive letter assignment. No more having the backup drive's letter jumping around.-40hz
Server headless - Caching disabled ... I'm thinking about filling the other USB parts with Silly Putty ... That'll keep the riff raff out.