Overview of creating disk sets
A Redundant Array of Independent Disks (RAID) lets you combine multiple disks so they work as one disk. 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 disk sets with both internal and external disks.
Striped RAID set (RAID 0)
You can combine multiple hard disks into a striped RAID set to provide faster disk access. For example, in a striped RAID set composed of three disks, data is distributed between all the disks and three disks write and read that data simultaneously, giving you faster access. A striped RAID set is especially useful for quickly accessing large files, such as digital video and database files. A striped RAID set works best if the disks are about the same size and are connected to your computer via fast connections.
If the disks are different sizes, a striped RAID set treats all the disks as though they are as large as the smallest one. If you have disks of different sizes, you might want to use a concatenated disk set, which doesn’t give you the speed advantage of a striped RAID set, but lets you use the full capacity of all its disks.
If the disks are connected to the computer with slow connections, such as USB connections, you might not get faster disk access with a striped RAID set. For best results, use internal disks or ones with fast connections.
Mirrored RAID set (RAID 1)
A mirrored RAID set can help protect your data against disk failure. Each disk in the set stores the same data, similar to having multiple backup disks. Your data is written on two or more disks at once, so if a disk fails or is disconnected, your Mac can still access the data from the other disks. If you reconnect a disk that was temporarily disconnected, the RAID set can rebuild the reconnected disk so that it contains the same data as the other disks.
You can also add spare disks to a mirrored RAID set. A spare disk isn’t used unless another disk fails or is disconnected. When a disk fails or is disconnected, the RAID set switches to use a spare and rebuilds it to contain the latest data. If you reconnect a disk after the RAID set replaced it with a spare, the reconnected disk is treated as a spare.
Even if you have a mirrored RAID set, you still need to back up your data regularly. Mirroring protects you from some types of hardware failure, but not from user errors or software corruption. If you delete a file, it’s deleted from the mirror as well. If software corrupts a file, it’s also corrupted on the mirrored disks.
Concatenated disk set (JBOD)
A concatenated disk set, also called “Just a Bunch of Disks” (JBOD), combines several smaller disks into a single large disk.
A concatenated disk set is helpful if you have a file, such as a database, that’s larger than any of your disks. You can use Disk Utility to add more disks to a concatenated disk set to increase its size.
You can also use a concatenated disk set to create a mirrored or striped RAID set with disks that are different sizes. Because mirrored or striped RAID sets store data most efficiently using disks that are roughly the same size, you might concatenate two smaller disks into a set that matches the size of another large disk, and then combine the concatenated set and the large disk into a mirrored RAID set. (If all the disks in the set are about the same size, consider using a striped RAID set, which lets you access your data more quickly.)
RAID disk sets cannot use some disk management features, such as FileVault disk encryption.
Note: 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.