MSA 2040 expand RAID 10 VDISK - help

 I am getting an error trying to expand a RAID 10 vdisk fron 4 to 8 disks. Any help appreciated I'm new to HP storage - and going off it rapidly!!

The GUI only lets you assign a max 4 disks to a RAID 10 vdisk on creation: is that the maximum allowed?

I cannot find this in a manual.

expand vdisk disks 1.17-20 dgA01

Error: The pool requested is of the wrong storage class.

thanks Rob