How do you know you got lucky with corrupted blocks?
mdadm doesn’t checksum data, and just trusts the HDD to either return correct data, or an error. But HDDs return incorrect data all the time, their specs even tell you how much incorrect data they’ll return, and for anything over about 8TB you’re basically guaranteed some silent corruption if you read every byte.
> and for anything over about 8TB you’re basically guaranteed some silent corruption if you read every byte
This is simply not true. I've been running bi-weekly ZFS scrubs on my file server which has grown from 80 TB to 200 TB of data over three years now with zero failed checksums. That is petabytes of reads with zero corruption. The oldest drives are nearing 5 years old (they were in a Btrfs Synology NAS before, again, zero failed checksums).
The wildly pessimistic numbers on the spec sheets are probably just there so they can deny warranty replacements by saying that some errors are in-spec.
Those specs aren't true, or at least the distribution is very far from uniform and they're failing to explain it even a fraction of the way. You don't get corruption that often.
mdadm doesn’t checksum data, and just trusts the HDD to either return correct data, or an error. But HDDs return incorrect data all the time, their specs even tell you how much incorrect data they’ll return, and for anything over about 8TB you’re basically guaranteed some silent corruption if you read every byte.