Add a disk to a disk set using Disk Utility on Mac
You can add a disk to a mirrored RAID set to replace a damaged disk or to extend the storage capability of the set. You can also add disks to a mirrored RAID set to serve as spares. A spare disk isn’t used unless another disk in the set fails, at which time the spare is rebuilt to hold data as a normal RAID disk member.
You can add disks to concatenated disk sets to increase access speed or increase the storage capacity of the set. You can’t add disks to a striped disk set.
Important: When you add a disk to a disk set, all the original files on the added disk are erased. Be sure to back up any important files on the disk before you add it to a set.
Connect the new disk to your computer.
In the Disk Utility app
on your Mac, select the disk set in the sidebar, then click the Add button 
.Choose Add Member or Add Spares to designate the disk as a standard member or spare in the set.
If you are adding a disk to a concatenated disk set, the Add Member and Add Spares options don’t appear.
Select the disk you want to add in the disk member list, then click Choose.
Click Add.