BEWARE: Using a RAID with a NAS/etc makes your *NAS* a potential single-point failure!
As most devices implement RAID in different ways, if you NAS dies you will likely be unable to read the data on your RAID HDs.
The solutions are:
1. Buy an identical backup NAS , or
2. Don't use RAID, but simply use a traditional backup system, possibly in combination with a (non-RAID) NAS...