per the link "Backup Exec does not support swapping different hard drives in an attempt to use different physical devices for the same backup-to-disk folder in Backup Exec".
You could use two usb drives and just do separate backups, it works fine.. but when you trying to backup for, lets say a week, and then switch out the hard drives, keeping the same drive letter, Backup Exec looks for the previously backed up media, when no found, it errors and you will basically have to reconfigure the backup job.. definitely was not what i was looking for...