[Twirrim]

Data Density is the biggest single driving need for storage when you get towards datacentre / cloud environments. You want as many TB per rack as you can possibly get, because your dominant cost over time is not the initial upfront capital + depreciation, it's the per-rack running costs.

S3, BackBlaze etc. all focus on cramming as many hard disks in to a single machine as they can do, without running in to other bottlenecks on the machine level (CPU, memory, NIC bandwidth, controller etc).

You very much want to get out of the RAID business in those environments too. Backblaze mention their use of Reed-Solomon which is fairly common on large scale storage, and moves you much closer to resiliency on an individual object basis, rather than thinking in terms of the entire drive.