I have wondered about system downtime or time operating in a degraded state.
My understanding is other than mirrored, RAID configurations may take a long time to rebuild on the larger drives and this is a contributing factor to why the highest sales volume of drives has been 'stuck' at 4TB (thus the lower $/GB price).
They don't use traditional RAID setups there. My understanding is they use a proprietary data encoding and distribution, which is more accepting of individual drive failures and reduces rebuild times. I believe I've heard they use something more like erasure coding rather than RAID-5.
My understanding is other than mirrored, RAID configurations may take a long time to rebuild on the larger drives and this is a contributing factor to why the highest sales volume of drives has been 'stuck' at 4TB (thus the lower $/GB price).