Expanding a Storage Pool by Replacing Disks in a RAID Group
You can increase the maximum storage capacity of a storage pool by expanding a RAID group in the pool. To expand the RAID group you replace one of the group's member disks with a higher-capacity disk, wait for the RAID group to rebuild, then repeat until all of its disks have been replaced. This operation can be performed while the storage pool is online and accessible to users.
The additional capacity from the new disks is added to the storage pool after the RAID group finishes rebuilding for the final disk.