Right, so I've now got 6TB of backup storage in my Synology NAS box. I'm going to use this as a backup for my unRAID NAS box. All I have to do now is figure out how to set rsync up to do the necessary stuff and then sit back and wait a few hours/days until it copies it all over.
The 2 NAS are located in different buildings to avoid any problems in case one goes missing or gets damaged! They are connected with 1GB ethernet.
The Synology, which I am using as the backup device, has 2 x 3TB disks in, which are set as RAID 0. I'm not too bothered if one of the disks fails in this box as it is only a backup of the main unRAID NAS anyway.