Hi,
I'm planning to build my own NAS and my main requirement is to be able to use mixed sized disks to build a starting RAID and then when I buy bigger disks, I simply upgrade the smallest one and voila! I have bigger array 
Drobo and Netgear's ReadyNAS currently have this feature in their product.
I'm currently experimenting with OMV in a VM environment to see whether it can do the same. I've installed OMV in a VM environment and I'm having trouble replicating this feature.
What I have
Disk A : 8GB
Disk B : 15GB
Disk C : 15GB
According to http://www.z-a-recovery.com/art-raid-estimator.htm I should be getting 16GB which is correct. I removed the 8GB and create an 10GB drive, so my new setup is below,
Disk A : 10GB
Disk B : 15GB
Disk C : 15GB
I've tried couple of combination but I simply cannot grow the RAID. This are the steps that I have done
1st attempt:
1.) Recover raid with the new disk
2.) Click Grow form web GUI - nothing happened
2nd attempt:
1.) Added 4th disk
2.) Click Grow form web GUI
3.) Remove 4th disk and added the new disk A which is 10GB.
4.) Tried to rebuild array with 3 disks. Not successful - I have issues.
I've very little knowledge with Linux so after those attempts, I hit the wall and couldn't progress. What I have done wrong?
According to the calculator with those combination I should be getting 20GB..
Cheers..
BC