By default, Data ONTAP adds new disks to the most recently created RAID group until it reaches its maximum size. Then Data ONTAP creates a new RAID group. Alternatively, you can specify a RAID group you want to add disks to.
When you create an aggregate or add disks to an aggregate, Data ONTAP creates new RAID groups as each RAID group is filled with its maximum number of disks. The last RAID group formed might contain fewer than the maximum RAID group size for the aggregate. In that case, any disks added to the aggregate are added to the last RAID group until the specified RAID group size is reached.
If you increase the RAID group size for an aggregate, new disks are added only to the most recently created RAID group; the previously created RAID groups remain at their current size unless you explicitly add disks to them using the -g option of the aggr add command.