I backup what kreij says, my nas at home is 2x 1tb drives in raid 1 I have that scheduled to do a backup to an external 2tb drive. The 80$ I spent on the external is easily justified if the nas ever kicks the bucket (remember a raid controller failure has a chance to take the drives with it) also I have it scan the nas first to ensure its virus/malware free before its backed up to the external.
There are a couple of things you can't help though.
1. data corruption, without a utility to scan for this or without you manually checking everything for it your scheduler will backup a corrupted copy. (this includes something that was corrupted from malware/viruses that have since been cleaned) A workaround for this is to have a backup solution that's larger than your NAS. This allows you to save multiple copies and run a file comparison program to ensure that corrupted data doesn't overwrite your master copy without interrupting an automatic schedule or making the backup take so long it times out or fails to finish before new data is overwritten by normal activity.
2. total destruction as in fire, earthquake, flooding, lightning, etc. If your solution is entirely held at one location, you're screwed in the event of a disaster. Many will cite an online backup solution as a way to avoid this, but upload speeds are prohibitive if you have a lot of data to backup, also online backup solutions don't allow you to use your own utilities to scan for data corruption of the backed up copy. Instead this makes it necessary to pick and choose what is most important and either keep that backed up to a thumb drive that's always with you or a separate backup that you do manually and then dump into a fire proof/flood proof safe (you should have one for your most important paper documents anyways). An online solution can be used for this important data, but you will need something to manually scan and compare the data for corruption before it overwrites it.
just food for thought. In the end I have no idea how important the data you're looking to back up is. Most of the time when you're running out of space its movies, music, or games and not the most important thing in the world. A 128GB thumb drive could be used for a "important data" backup and then you could leave the NAS as is without the need to back up the whole thing, just the things that matter to you (pictures, self taken videos, important professional and personal documents, etc. Most will fit easily on 128GB.)