Error (/var/log/messages):

raid1: raid set md0 active with 2 out of 2 mirrors
md: kicking non-fresh <DEVICE> from array!

 

Solution:

This can happen after an unclean shutdown (like a power fail). Usually removing and re-adding the problem devices will correct the situation:
 
/sbin/mdadm /dev/md0 --fail /dev/sda5 --remove /dev/sda5
/sbin/mdadm /dev/md0 --add /dev/sda5

 

Verification:

root@r122:~> cat /proc/mdstat