Just as a workaround, there are a ton of backup programs out there, a lot of them free, that will do the job for you with the hardware as it is. I mention this as i always run backups and i have never run into a program that wouldnt back up to a standard disk! So i think you should dump your software, you can google a replacement for free! Or just wait a bit, me or others will give you some examples of software you can get free.
Some hardware setups work differently, some set up as a removable (floppy drive), some set up as a hard drive.
If i had run into that then i would know what to tell you but i havent and i have used quite a few different backup progs. You could try a different usb box or even a firewire box and see if they set up as a removable instead of a hard drive. Bt again, i would simply get new software.
What software are you using and i would imagine that you have checked to see if there is a setting that would enable it to be used, so i wont go there.
Good advice + great people = tek-tips