But the answers they do have are worth considering. 75 PB is a large sample.
Backblaze has currently a total population of approximately 27,000 drives. Five years ago that number was about 3000. However, they've kept track of all the drives and found some interesting things that at least partly contradict earlier research done with Google by Carnegie Mellon university. (See Everything you know about disks is wrong.)
They measured annual failure rates. If you have 100 drives for a year and five of them fail that is a 5% annual failure rate.
In the first 18 months drives failed at the rate of 5.1 percent per year. For the next 18 months drives failed at the rate of about 1.4 percent per year. But after three years failures went up to 11.8 percent per year.
While that sounds bad the good news is that after five years almost 80 percent of drives are still working. Which explains why they don't actually have an answer to the question how long drives last.
But extrapolating from their experience they believe the median lifespan of a consumer drive will turn out to be six years.
The Storage Bits take
It is refreshing for a large-scale user of hard drives to break the industry code of silence and tell us their experience with a large population of disks. Lots of companies have this information - I'm looking at you, Google and Amazon - and simply refuse to share it.
But, if like me, you only buy a few drives a year, this information may not apply to you. You can get drives from a batch that were marginal or dropped during shipping or poorly handled and you might see several drive failures from single batch.
Or you might have drives that last for 10 years. The important thing to keep in mind is that in five years you can expect at least one in five drives to fail.
The bottom line: backup, backup, backup. Accept no substitutes.
Comments welcome, of course. Backblaze would like you to back up with them. I have no commercial dealings with Backblaze, but I like what they do. Read their entire blog post here.