Add a new drive (sdf) to raid array md1:-
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

mdadm --add /dev/md1 /dev/sdf1
mdadm --grow /dev/md1 --raid-devices=4

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

fsck.ext3 /dev/md1
resize2fs /dev/md1


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