There are pro's and con's with 'mirrored' drives, which is the simplest form of redundancy...

In theory, when one drive fails, the other still has all your stuff.

Other people say that the more likely scenario is that a software error will simply be replicated and you've still lost the lot. I'm inclined to think it is more likely to occur in a mirrored RAID system drive.

I'm happy to rely on a NAS with mirrored drives (Network Attached Storage) for at least one of my backup devices, and USB drives for others.

Interestingly, I have a couple of LG-NAS (LG as in the manufacturer) units with pop-up USB slots... insert a memory stick and it will back it up for you. I've yet to find out how to restore, though...! Maybe it's a real example of John's 'Write Only Memory'...!!