The answer may simply that as described on https://raid.wiki.kernel.org/index.php/Scrubbing_the_drives " If a read error is encountered, the block in error is calculated and written back. If the array is a mirror, as it can't calculate the correct data, it will take the data from the first (available) drive and write it back to the dodgy drive. Drives are designed to handle this - if necessary the disk sector will be re-allocated and moved. " However, I am unsure whether this is the case here, as I would expect * dmesg to mention that md took this action * mdadm to send me an email of this event, as it is an indication that a disk needs replacement
