[trapexit]

Not as unlikely as many people would like to think! If the two drives are from the same production lot, they may suffer from a common manufacturing defect. And because they are in the same chassis, if a server fan fails, both drives may subsequently fail due to thermal damage within a very short interval.

Even if they don't fail simultaneously, the mirror drive may fail (or even more likely) have read errors or flipped bits that will corrupt the restore or render it impossible.

Personally, I don't place much trust in any RAID configuration other than RAIDZ2 (ZFS; you can lose two drives and still recover all your data; every block is checksummed to avoid reading or restoring corrupted data).

But even ZFS can't protect you against accidental deletion, fire, theft, or earthquake.

[mietek]

There's RAIDZ3 now!