[Zubersoft]

If you go to Settings->Backup and Restore->Backup Library, tap on the folder icon, then tap on the SD card icon at the top of the file browser, it should list all detected removable storage. Do you not see the USB show up in that list? Can you try just creating the backup file in the internal storage, and then use a file manager to move the file over to the USB stick? I imagine that would work equally as well. I'm not sure why you are seeing issues backing up to Dropbox though. How large is a typical backup file for your library and how much storage space do you have available in your Dropbox account?

Mike