> The previous person seems to claim it meant the disk was dead (i.e. no read works)
He made no such claim.
> while you seem to claim that it means an error caught by low level formatting.
A: There is no such thing as low level formatting in a modern drive, and B: No I don't. I said he should do a full disk read. Not format.
The SMART built in self-test does a full read of the drive, not write.
> Those scrubs do nothing to catch errors that the drives do not report such as misdirected writes.
That's only true with RAID 5. Ever other RAID level can compare disks and check that the data matches exactly. The Linux md software RAID does that automatically if you ask it to check things, and then it will report how many mismatches it found.
If you look he wrote: "I had to drop back to raid 5". He had a better level of RAID before, with multiple disk redundancy, that allows the RAID to check for mismatches and even correct them.
But because he never scheduled full disk reads the RAID never detected that many of the drives had problems.
> Consequently, there is no correct way to setup RAID in a way that makes dats safe.
That is not correct. The only advice I would give is avoid RAID 5. The other levels let you check for correctness.
> A check summing filesystem such as ZFS would handle this without a problem though.
Only if A: you actually run disk checks, B: and only if ZFS handles the RAID!!! ZFS on top of RAID will NOT detect such errors 50% of the time (randomly depending on which disk is read from).