Redundant Array of Independent Disks (RAID)
You can combine multiple disks into one disk set—called a Redundant Array of Independent Disks (RAID)—and they will work together as one. Depending on how you combine the disks, the RAID set can protect your data against hardware failure, speed up access to your data, or increase your storage capacity. You can use Disk Utility to create several different types of RAID sets with your internal disks and connected external disks.
RAID disk sets cannot use some disk management features, such as FileVault disk encryption.
If you have a Mac Pro with a Mac Pro RAID card, use RAID Utility. It uses the RAID card for better performance and to create more types of RAID sets.