Every night apart from friday i run an incremental backup to a rotation of usb hard drives. On friday i run a full backup.
The user in charge of the drives swaps the drive every morning in the same manner.
On occasions the drive sits at offline and the backup fails to run even though the drive is viewable and browseable in "my computer".
If every night before the backup runs and make the drive online and scan the drive in backup exec it never fails.