On Thu, 10 Dec 2009 13:54:29 +0530
[off-list ref] wrote:

Neil,

I get only the following message under /var/log/

[root@NAS00226b1755a0 ~]# mdadm /dev/md0 --grow --add /dev/sata3 
mdadm: Cannot add new disk to this array

Dec  9 20:37:58 NAS00226b1755a0 kernel: md: export_rdev(sdc)

[root@NAS00226b1755a0 ~]# mdadm /dev/md0 --grow --add /dev/sata4
mdadm: Cannot add new disk to this array

Dec  9 20:38:30 NAS00226b1755a0 kernel: md: export_rdev(sdd)

Thanks - that narrows it down quite a bit.

I think that if you run mdadm under 'strace' you will find that the ioctl
fails with ENOSPC.
It seems that growing a linear array didn't work reliably until 2.6.22.

You need 

  commit a778b73ff793d34c0082f76110f73a7754915067

to make it work properly.

NeilBrown