RAID
From Atomicorp Wiki
Rebuilding a failed array.
1) the old state
/dev/md0 is /dev/hda1 and /dev/hdc1
has /dev/hda failed
2) replace /dev/hda
3) partition /dev/hda to create RAID containers like hdc. They must be the same size as the partitions on the other disk, and of type fd (Linux raid autodetect)
4) mdadm --add /dev/md0 /dev/hda1
5) To monitor progress: cat /proc/mdstat
6) Optional: regrub the disk if this is /boot