Rocel ,
I am afraid SME 7.3 is a bit old for your problem...
Kernel 2.6.9-67.0.7.EL
mdadm - v1.12.0 - 14 June 2005...you need at least kernel 2.6.17 (if memory serves me right) (also mdadm current is at 2.6.1)
otherwise it's a easy as:
(assuming you add identical formatted
/dev/sdd _and_ you used '
nospare nolvm')
mdadm --add /dev/md3 /dev/sdd3
mdadm --grow /dev/md3 --raid-devices=4Will take "couple of hours" to reshape the array.
cat /proc/mdstat[>....................] reshape = 0.0% (xxx/yyyy) finish=1234.5min speed=6789K/sec
The filesystem then needs to be expanded to fill up the new space (have a look
here):
fsck.ext3 /dev/md3
resize2fs /dev/md3WARNING: You could
(of course) use a LIVECD (Knoppx?) for this...

...but it sure is dangerous and you'd better know what you do.

Regards
Reinhold