alt Hacker NewsWere those arrays doing regular scrubs, so that they experience rebuild-equivalent load every month or two and it's not a sudden shock to them?
If your odds of disk failure in a rebuild are "only" 10x normal failure rate, and it takes a week, 5 disks will all survive that week 98% of the time. That's plenty for a NAS.
If the drives are the same age and large parts of the drive haven't been read from for a long time until the rebuild you might find it already failed. Anecdotally around 12 years ago the chances of a second disk failing during a raid 5 rebuild (in our setup) was probably more like 10-20%