added third disk to raid1 (linux raid pt. 2)
this was very similar to the last post, but instead of just adding another disk, i had to also grow the array so that the new disk would sync up instead of remain a spare. that was accomplished with the following command:
$ sudo mdadm --grow --raid-disk=3 /dev/md0
now everything is syncing up and i should have a little better read speeds. yay!