As you have probably noticed the Backup Wizard in SBS console allows you to use only local external or internal disks as a destination.
You can also backup to Network Share, and that is what we demonstrate in the following screencast. This approach is especially useful when you consolidate different backups to a network share and then transfer them to a Tape backup.
Bear in mind that when you backup to a network share, you cannot create VSS snapshots on the destination drive. Each full backup will overwrite the previous, unless you specify a different folder.