Add a disk to a disk set
You can add a disk to a mirrored RAID set to increase your protection or replace a damaged disk. You can also add disks to a mirrored RAID set to serve as spares. A spare disk isn’t used unless another disk of 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 a set. You can’t add disks to a striped disk set.
Important: When you add a disk to the 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 the set.
Connect the new disk to your computer.
In Disk Utility, 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.