I bought one of the original 5-big drives and one drive soon failed. I was shocked to find that there's no warranty replacement for individual drives. You send the entire RAID back and they give you an entirely new RAID. That wasn't the solid uptime I was hoping to get from a RAID-5 configuration.
I guess it's still better than Drobo showing all lights green while it's losing everything to I/O errors. (maybe Drobo just fixed this bug)