egork's Journal: replace a disk in mdadm raid array on ubuntu

# mdadm --manage /dev/md0 --fail /dev/sdb2
# mdadm --manage /dev/md0 --remove /dev/sdb2

Past that point, its just a matter of powering off and replacing the drive at your earliest convenience. Once you got a new drive, pop it in, boot the system up, clone the partition table, and add the new partition to the array:

# sfdisk -d /dev/sda | sfdisk /dev/sdb
# mdadm --manage /dev/md0 --add /dev/sdb2

http://erikugel.wordpress.com/2010/04/11/setting-up-linux-with-raid-faster-slackware-with-mdadm-and-xfs/