Replacing a faulty drive in an mdadm managed RAID array.

1. Use 'mdadm –manage /dev/md0 -r /dev/sdd' to remove the drive that was marked as faulty from the array.
2. Power down and replace the drive with a good drive.
3. Power up and set the partition table on the new drive to match those of the other drives in the array. Here we used 'sfdisk -d /dev/sda | sfdisk /dev/sdd'.
4. Add the proper partition on the new drive into the array, 'mdadm –manage /dev/md0 -a /dev/sdd2'
5. Sit back and wait for the recovery to happen, you can 'watch cat /proc/mdstat' to watch its progress; you should see something like:

Ref: http://www.linuxquestions.org/questions/linux-server-73/mdadm-raid-5-single-drive-failure-644325/#post3173822

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