ReadyNAS NV+ stuck on "Booting Please wait"

I thought the purpose of the thing was that if one disk failed (in this case disk 3) then I can put in a fresh one and it'll rebuild itself? I've been careful to not have two disks missing at once. I'll run the diags, but I know at least that one disk is dead. What I don't understand is why a disk failure can result in the thing not booting, isn't the point of this that it should fail gracefully? Otherwise it's not much more use than a single disk alone, as a failure of a single disk in this case has rendered my data inaccessible.