For redundancy, you might consider using an online service. I use Amazon AWS but there are several other options. I think QNAP and Synology both have apps available to automate the process.
The up-side is that it's off-site storage. If something other than drive failure causes you to lose your data (fire, theft, flood, electrical, etc), you still have a copy of your data. The monthly fee is based on storage size but relatively inexpensive. It also frees up a drive on your NAS for more local storage.
The down-side is that download speeds are slow. It would take you a few hours to days to download your data again if you've lost it, but tha'ts what allows storage providers to keep costs down.
- Garrett