On Wed, 9 Dec 2009 20:31:54 +0530
[off-list ref] wrote:

Hi,

I am trying to add a disk to a Linear Raid which was created with 2
disks.
We use linux-2.6.18 and mdadm-2.6.4.
I guess these support the above mentioned disk addition.

Yes, should work.  This functionality was introduced in 
2.6.18.

root@NAS00226b1755a0 ~]# mdadm /dev/md0 --grow --add /dev/sata3  

This is the correct usage but,
   

mdadm: Cannot add new disk to this array

This indicates that something went wrong.

Do you get any messages in the kernel logs at this point?

NeilBrown