The normal failure mode for a disk is to return an error or nothing, not random bits. A straight mirror or similar structure can’t defend against that, you need

The normal failure mode for a disk is to return an error or nothing, not random bits. A straight mirror or similar structure can’t defend against that, you need checksums. If you don’t have them the filesystem will mostly break and the files you get are partly filled with the random bits. It’s kind of impressive though that the filesystem doesn’t crash, many filesystem drivers trust the disks and don’t defend against corrupted or manipulated data.