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.