Your usage is different than mine: I have a 18TB disk in my desktop tower so all my data is always available and have a second tower with 10x2TB disks in raid 6

Your usage is different than mine: I have a 18TB disk in my desktop tower so all my data is always available and have a second tower with 10x2TB disks in raid 6 for backups (all my old disks). I switch on the second tower once a week and rsync everything from my desktop. I don't have a solution if both fail at the same time but the probability of it happening is low enough that I accept the risk. Ideally I would need a third backup rig somewhere outside my house, but I can't afford this level of safety.