> If I were Backblaze, I'd design the system to work equally well with 40 drives as with 45, and consider just leaving bad drives in until a handful of them nee

> If I were Backblaze, I'd design the system to work equally well with 40 drives as with 45, and consider just leaving bad drives in until a handful of them need replacing in a chassis. And the replacement be basically automatic. They designed their own replication scheme which allows quite a bit of disk to fail. Considering that, using RAID would be absurd so I'm pretty sure they are all configured as JBOD, thus allowing this kind of replacement.