ok. Thanks.
The disks are all the same. 15k 300GB.
The reason I created a Raid 10 diskgroup at the beginng was to have the best perfomance on SQL end Exchange transactions, but I'm not sure if it is noticable here in this setting.
I think I'll go with your advise and create two Raid6 diskgroups then wtih the 8 + 10 disks. I need the space...
If I am going to add two more disks to create to even dsikgroups with 10 disk each, I will have to use the two Global spares disk in the same diskshelf. If not the Diskgroup will span over two diskshelf, and this is not recomended..
But I can ask the customer to buy two more disk if it will be reasonable