> Based on Backblaze’s stats, high-quality disk drives fail at 0.5-4% per year. A 4% risk per year is a 2% chance in any given week. Two simultaneous failures would happen once every 48 years, so I should be fine, right?

Either I misunderstood or there are some typos but this math seems all kind of wrong.

A 4% risk per year (assuming failure risk is independent of disk age) is less than 0.1% by week. A 2% risk per week would be a 65% risk per year!!

2 simultaneous failures at the same week for just 2 disks (again with the huge assumption of age-independent risk) would in the order of magnitude of less than 1:10^6 , so more than 20k years(31.2 k years tbc)

Of course you either change your drives every few years so the age-independent AFR still holds or you have to model the probability of failure using some exponential distribution like Poisson's. Exercise for the reader to estimate the numbers in that case.