I was initially backing up to two different NAS devices (Yellow Machine & H.P. MediaSmart Server). When my died on me, I had to get it repaired, but once it was back on-line I added a Drobo and my third NAS device. I now back up everything to all three.
Overkill? Maybe, but I no longer worry about my thousands of photo images any more.
I should mention, I don't keep any of my archived files on my main working computers.