I mostly agree with this. That's why I asked about GP's motivation.
I suspect, though, without any actual data to back this up, that the drives that go into (low-cost) external HDD enclosures have lower MTBF ratings than the desktop-oriented internal drives. I think that the usage pattern for an external HDD is very different in terms of longevity in operation than desktop hard drives (and these have lower guarantees than eg. server-grade SATA drives).
If I was a manufacturer of such enclosures I'd be willing to buy cheaper drives with lower promised MTBF for embedding them in enclosures that stay off most of the time.
So, even if they're the same, I guess the manufacturer performs some tests and sells drives that fail some non-critical ones at a lower price (pretty much what Intel and AMD do with their chips - those that can't sustain the highest frequencies are downgraded to clock rates that are stable).
edit: The comment from @dsr_ tends to support my hypothesis, although I haven't looked at the differences in guarantees of external enclosures, it's true that internal desktop HDDs can have up to 5 years of guarantee (i.e. WD's Black series has a 5 years guarantee).
But then Backblaze's use of HDDs is not the same as an HDD stuck in a desktop machine and used as a primary/secondary drive.
It's why their restore isn't instantaneous. You have to ask for the files and they'll let you know when they're ready, this takes time (up to several hours).
AIUI they essentially fill up each Pod with incoming data and, when it's full it goes into a low power mode where they spin down all of the disks in that Pod.
Restore requests (from many users) will usually span quite a few Pods so they batch up requests for each Pod so that can minimise the number of times that drives in a Pod need to be spun up to copy off files. Rather than spinning up and down once every 10 minutes to service 36 requests over 6 hours they wait 6 hours, spin up once and service all 36 requests for that pod in one go. (Figures are for example only.)
(Small restores may be instantaneous if the Pod is still being filled or just happens to be spun up for restore soon. Large restores will more than likely span multiple pods and so there's more of a delay until all parts of the restore have been obtained from all of the pods involved.)
If anything this maps pretty well to the expected usage pattern of an HDD in an external enclosure as many of them are used for infrequent backups rather than extending constant use storage. It's how I expect Backblaze will be hoping to get much more than the usual MTBF out of their drives they have purchased.
HDD guarantees may be time based, but HDD failure rates are linked much more closely with usage patterns; number of spin-ups/downs, number of seeks, number of sector writes, etc. A drive in constant use on a desktop machine is going to be hammered a lot more than one that's essentially written to just once (as the pod fills up) and then spun up every now and again for partial reads.
If thats the access pattern, couldn't they just use tape? Of course you can't easily bootstrap an automated warehouse for these things, but once you have it, you enjoy the drastically lower prices and better availability of tape.
(I know nothing about using tapes to seriously store data, so this is conjecture.)
I suspect, though, without any actual data to back this up, that the drives that go into (low-cost) external HDD enclosures have lower MTBF ratings than the desktop-oriented internal drives. I think that the usage pattern for an external HDD is very different in terms of longevity in operation than desktop hard drives (and these have lower guarantees than eg. server-grade SATA drives).
If I was a manufacturer of such enclosures I'd be willing to buy cheaper drives with lower promised MTBF for embedding them in enclosures that stay off most of the time.
So, even if they're the same, I guess the manufacturer performs some tests and sells drives that fail some non-critical ones at a lower price (pretty much what Intel and AMD do with their chips - those that can't sustain the highest frequencies are downgraded to clock rates that are stable).
edit: The comment from @dsr_ tends to support my hypothesis, although I haven't looked at the differences in guarantees of external enclosures, it's true that internal desktop HDDs can have up to 5 years of guarantee (i.e. WD's Black series has a 5 years guarantee).