( ahhh I deleted my post before I saw the response )
From the numbers he cited I got the impression that the drives they are using would fail 3+ times more often than enterprise drives (lower MTTF + speced at low temps). The cost of replacing the drives (even if they are 1/2 the price) would make the system uneconomical.
Also: because of their case design (non-hot-swappable) the cost of human replacement is higher too.
Thus they must have a low usage rate to have a low failure rate for their system to make business sense.
I think their use case is sufficiently a-typical to get away with this.
They're primarily write-only, only when a customer retrieves their data does stuff get read.
If they spin down the drives when the volume is not being accessed the mttf for a single drive goes up a lot, possibly beyond the point where it matters.
Given proper redundancy and a adequately read/write unbalance, it's safe to power-down drives during read-heavy cycles and only power them up when you have some redundant data to write.
But you will have to manage the cycles for redundancy, lifetime and power savings. The software to do it must be really clever.
Or even to power down the entire pod if the data on it is quiescent. If you managed the fill and aging properly, this wouldn't even be hard to do -- as a backup service, they're write-heavy, which gives them a lot of flexibility that other applications don't have.
From the numbers he cited I got the impression that the drives they are using would fail 3+ times more often than enterprise drives (lower MTTF + speced at low temps). The cost of replacing the drives (even if they are 1/2 the price) would make the system uneconomical.
Also: because of their case design (non-hot-swappable) the cost of human replacement is higher too.
Thus they must have a low usage rate to have a low failure rate for their system to make business sense.