I set up backups in 2008 R2 to backup to a USB drive, and the backup program formatted the drive so it doesn't appear with a drive letter.

Rotating to the next of four backup drives breaks the backups because it has an NTFS partition on it. I don't see a way to prepare each of my USB drives to be used for backups. What kind of formatting is the backup program expecting to find and write to and how do I ensure my other USB drives can be rotated?

Thanks.