BackUp NAS

There's a few ways you could do it, but the easiest would proably be via nfs.

On the backup server setup an export in /etc/exports allowing the internal ip of you main server to mount it. On your main server put something in /etc/fstab indicating that it should mount the nfs drive (i.e. hostname:/path /backup nfs defaults 0 0 assuming you have the backup in /etc/hosts). There's an option in cpanel backup where it can mount and unmount the backup drive after it's done, which may cut down traffic/make the data a bit more secure.