4D Backup can only backup to drives that are mounted and currently running. A drive that is mounted can be seen by a user in the Finder on the Macintosh or in Explorer on Windows. 4D Backup assumes the drive to be currently running. If 4D Backup encounters a drive that is not currently running, ie the platters are not spinning, it usually does not handle the situation very gracefully. Drives, like Iomega's Jazz drive that automatically spin down the drive when not in use, may cause problems when 4D Backup tries to start a scheduled backup.
Some devices like tape drives are usually not mounted devices. So both you as a user and 4D as a backup will not have access to a tape drive unless the tape drive is mounted to your machine. There is at least one program for the Macintosh that mounts tape drives like a disk called DeskTape from Optima Technology.