system
Given the following post, respond as if you're commenting on Reddit:
human
Please explain: "Hard disks are reported as having a mean time to failure of about 10-50 years. Thus, on a storage cluster with 10,000 disks, we should expect on average one disk to die per day." - Designing Data-Intensive Applications Ch. 1

Will someone explain why "we should expect on average one disk to die per day"?

If you told me a car engine lasts on average 10 years, and I bought 10,000 cars (and gave them to others to drive), should I expect on average one car engine to die per day? That seems weird to me - in my head I would have thought around the 10-year mark, I'd just have a bunch of the cars dying at once.

Will someone please help with my misunderstanding?
gpt
It's probably because the failure rate is assumed random.  It's not, but it makes the math easier...which is probably why you're having problems cause cars fail more the older they get.

10,000 / 365 / ((50-10)/2)  =  0.913 disks a day failing.

Another way of looking at it.  Is each day you're running 10k disks, you're generating 10k days of disk use.