This post will cover how to replace a failed disk in a MD RAID1.

Mark the disk failed

mdadm --manage /dev/md0 --fail /dev/sdb1
Verify that this disk has been marked as failed.

    # cat /proc/mdstat
    Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
    md0 : active raid1 sda1[0] sdb1[2](F)
        976773168 blocks [2/1] [U_]

Remove the disk from the RAID

mdadm --manage /dev/md0 --remove /dev/sdb1

Copy the partition

sfdisk -d /dev/sda | sfdisk /dev/sdb

Add the disk to the RAID

mdadm --manage /dev/md0 --add /dev/sdb1

Verify!

watch cat /proc/mdstat