Add a new drive (sdf) to 4 disk raid array md0:-
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

fdisk the new disk and make a partition of type fd (Linux raid autodetect)
then add and grow the array:

mdadm --add /dev/md0 /dev/sdf1
mdadm --grow /dev/md0 --raid-devices=5

The filesystem the needs to be expanded to fill up the new space.

cat /proc/mdstat to find the new block count, eg
	29302876116 blocks level 5, 64k chunk, algorithm 2 [5/5] [UUUUU]

fsck.ext3 /dev/md0
resize2fs /dev/md0 29302876116


Credits: Ferg (http://scotgate.org/?p=107)