[CentOS] Cannot re-make a software raid pair

Apologies if you get this twice - the first one didn't seem to make it... 

Hi Guys,

I have just replaced a faulty Max...woah, wait...this one's a Seagate... IDE
hard disk but I cannot remake the software raid pair.

The currently running disk is hda and I am trying to add back hdg - both are
master drives on separate controllers.

I have run fdisk on hdg and created the same partition structure as had and
changed their types to 'fd' but when I try and rebuild the array I get:

mdadm /dev/md0 -a /dev/hdg1

mdadm: hot add failed for /dev/hdg1: Invalid argument

If I look at /var/log/messages, I have:

Oct  5 16:04:39 petdoctors kernel: md: could not bd_claim hdg1.
Oct  5 16:04:39 petdoctors kernel: md: error, md_import_device() returned
-16

>From other postings, this seems to imply the hdg1 partition is mounted but I
can find no evidence of this (df or cat /proc/mounts or lsof). I have done
sone googling and found others having fun with this but no specific
solution.

Anyone?

Thanks

Nigel Kendrick