Replacing a failed drive in an md raid1/raid10

• Check if the kernel already removed the faild drive from the arrays: # mdadm --detail /dev/md0
  • If not, remove it by hand: # mdadm /dev/md0 --remove /dev/sda1
  • If in an array a partition of the failed drive is not marked as faulty, before removing run # mdadm --manage /dev/md1 --fail /dev/sda2
• Insert a new physical disk
• Copy the partition table to the new drive from its mirror:
  • For gpt partition table:
    • # sgdisk -R /dev/sda /dev/sdb (from sdb to sda)
    • # sgdisk -G /dev/sdb (to make the new disk's GUID different the mirror's one)
  • For mbr partition table: # sfdisk --dump /dev/sdb | sfdisk /dev/sda
• Add the new drive into the arrays: # mdadm /dev/md0 --add /dev/sda1
• Observe the mirroring process with $ watch cat /proc/mdstat

Ref: http://wiki.hetzner.de/index.php/Festplattenaustausch_im_Software-RAID/en