I've considered getting a commercial RAID NAS, e.g. QNAP or Synology but hesitate because I want to be absolutely confident regarding disaster recovery. I'd prefer not to be at the mercy of the manufacturer.

Suppose I've been a fool and not remembered that RAID is not a backup. One day a disk in my RAID 5 or 6 array fails, the box itself breaks, and I can't buy the same box again. Are there open source tools that can reliably rebuild my data on a new drive/array?

This is why I want to avoid drobo, but would welcome suggestions on good manufacturers or principles to make disaster recovery simple. E.g. what if I had a 4 bay NAS and set up 2x RAID 1 mirrors which were presented as one drive. In my scenario above would I be able to just copy directly off the healthy drives once plugged into a computer?