A NAS is not a backup device.

To be honest, it seems rather silly to go this deep into probability calculations when your N = 4 (hard drives). I can present numerous wildly-differing anecdotes about how single hard drives I have owned have fared. I have some drives that are 20 years old that still apparently work fine. I've had brand new drives silently corrupt sectors after less than a year in use. Trying to put hard numbers to the probability of failures is a fool's errand. Better to simply assume that components will fail over time, and architect your system to handle those failures gracefully.

That said, I actually agree with much of the final architecture decisions made by the author. BTRFS raid1 with an offsite copy in a geographically distant area is a pretty good solution for the average person's personal documents. Possible improvements would be:

- Multiple offsite copies in different locations

- Hardware diversity (differing models of HDD, host PC, power supplies, etc.)

- Staggered hardware upgrades on a 5-10 year cycle

- Cold storage (offline) copy

off topic but I really like the design of the site