Can come handy when attempting to correct unreadable sectors on a RAID disk. Eg. if your RAID1 array is built around /dev/sda3 and /dev/sdb3 and you had unreadable sectors reported by SMART on /dev/sda (Current_Pending_Sector), but got no error during the SMART extended self test, you might want to resync the /dev/sda partitions on /dev/sdb as follows:
mdadm --fail /dev/md2 /dev/sda3
mdadm --remove /dev/md2
/dev/sda3
mdadm --add /dev/md2 /dev/sda3
