raid howto is located in Crypto

https://raid.wiki.kernel.org/index.php/RAID_Recovery

if a raid is degraded, save superblock infos first:

mdadm --examine /dev/sd[abcd]1 > /etc/_raid/mdadm--examine_2013-03-10

check, whats up:

mdadm --detail /dev/md1

re-add a removed device:

mdadm  /dev/md1 --add /dev/sda2

watch revovery:

watch cat /proc/mdstat