Use Disk Utility to Create a RAID 1 (Mirror) Array

A RAID 1 array contains the same set of data on two or more disks. Also known as a mirrored array, RAID 1 is one of the Redundant Array of Independent Disks (RAID) levels supported by OS X and Disk Utility. In a RAID 1 array, you assign two or more disks as a mirrored set. After you create the mirrored set, your Mac sees it as a single disk drive.

How Your Mac Works With a RAID 1 Array

When your Mac writes data to the mirrored set, it duplicates the data across all members of the set to ensure that the data is protected against loss if any hard drive in the RAID 1 set fails. As long as a single member of the set remains functional, your Mac continues to operate normally, giving you complete access to your data.

You can remove a defective hard drive from a RAID 1 set and replace the hard drive with a new or repaired hard drive. The RAID 1 set then rebuilds itself, copying data from the existing set to the new member. You can continue to use your Mac during the rebuilding process because it takes place in the background.

Why RAID 1 Isn't a Backup

Although it's commonly used as part of a backup strategy, RAID 1 by itself is not an effective substitute for backing up your data with Mac backup software or other methods.

Any data written to a RAID 1 set is immediately copied to all members of the set; the same is true when you erase a file. As soon as you erase a file, that file is removed from all members of the RAID 1 set. As a result, RAID 1 does not allow you to recover older versions of data, such as the version of a file you edited last week.

Why Use a RAID 1 Mirror

Using a RAID 1 mirror as part of your backup strategy ensures maximum uptime and reliability. You can use RAID 1 for your startup drive, a data drive, or your backup drive.

What You Need to Create a RAID 1 Mirror

To create a RAID 1 mirror for your Mac, you need a few basic components.

• OS X Leopard (10.5) through OS X Yosemite (10.10).
• Disk Utility, which is included with OS X.
• Two or more hard drives. The process of creating RAID 1 mirrored sets erases all the data on the hard drives. Using hard drives that are the same make and model is recommended but not required.
• One or more drive enclosures. Mac Pro users may have internal drive bays available. Everyone else needs one or more external drive enclosures. If you use multiple drive enclosures, they should be the same make and model or at least have the same type of interfaces, such as FireWire, USB, Thunderbolt, or SATA.

The process of creating a RAID set is relatively simple and doesn't take much time, but erasing the drives in the RAID set using the Zero Out Data option is a time-consuming process that ensures maximum reliability.​

Erase the Drives

The hard drives you use as members of the RAID 1 mirror set must first be erased.

Because you are building a RAID 1 set so that your data remains always accessible, take a little extra time and use one of the Disk Utility Security Options, Zero Out Data, which erases each hard drive.

When you zero out data, you force the hard drive to check for bad data blocks during the erasure process and mark bad blocks as not to be used. This decreases the likelihood of losing data due to a failing block on the hard drive. It also increases the amount of time it takes to erase the drives from a few minutes to an hour or more per drive.

Erase the Drives Using the Zero Out Data Option

Erase each of the drives you plan to use in the RAID 1 mirror set.

1. Connect the hard drives you intend to use to your Mac and power them up.

2. Launch Disk Utility, located at Applications > Utilities.

3. Select the hard drive you are going to use in the RAID 1 mirror set from the list in the left pane. Select the drive, not the volume name that appears indented under the drive's name.

4. Click the Erase tab.

5. From the Volume Format drop-down menu, select Mac OS X Extended (Journaled) as the format to use.

6. Enter a name for the volume.

7. Click Security Options.

8. Select the Zero Out Data security option and then click OK.

9. Click Erase.

10. Repeat steps 3 through 9 for each additional hard drive that will be part of the RAID 1 mirror set. Give each hard drive a unique name.​

Create the RAID 1 Mirror Set

After you erase the drives you plan to use for the RAID 1 set, you're ready to build the mirror set.

1. Launch Disk Utility if the application is not open.

2. Select one of the hard drives you plan to use in the RAID 1 mirror set from the Drive/Volume list in the left pane of the Disk Utility window.

3. Click the RAID tab.

4. Enter a name for the RAID 1 mirror set. This is the name that displays on the desktop.

5. Select Mac OS Extended (Journaled) from the Volume Format drop-down menu.

6. Select Mirrored RAID Set as the Raid Type.

7. Click Options.

8. Set the RAID block size. The block size depends on the type of data you plan to store on the RAID 1 mirror set. For general use, consider a block size such as 256K or larger to optimize the performance of the RAID.

9. Select Automatically Rebuild RAID mirror set for the RAID 1 mirror set you are creating to automatically rebuild itself if the members of the RAID become out of sync.

   Automatic building may not be a good idea when you use the RAID 1 mirror set for data-intensive applications. Even though it's performed in the background, rebuilding a RAID mirror set uses processor resources and may affect your use of the Mac.

10. Make your choices on the options and click OK.

11. Click the + (plus sign) to add the RAID 1 mirror set to the list of RAID arrays.​

Add Slices (Hard Drives) to Your RAID 1 Mirror Set

With the RAID 1 mirror set now available in the list of RAID arrays, it's time to add members or slices to the set.

1. Drag a hard drive from the left pane of Disk Utility onto the RAID array name you created. Repeat this for each drive you want to add to the RAID 1 mirror set. A minimum of two slices, or hard drives, is required for a mirrored RAID. After you add the hard drives to the RAID 1 mirror set, you are ready to create the finished RAID volume for your Mac to use.

2. Click Create.

3. A Creating RAID warning sheet drops down to remind you that all the data on the drives that make up the RAID array is about to be erased. Click Create to continue.

During the creation of the RAID 1 mirror set, Disk Utility renames the individual volumes that make up the RAID set to "RAID Slice." It then creates the RAID 1 mirror set and mounts it as a normal hard drive volume on your Mac's desktop.

The total capacity of the RAID 1 mirror set you create is equal to the smallest member of the set, minus some overhead for the RAID boot files and data structure.

You can now close Disk Utility and use your RAID 1 mirror set as if it were any other disk volume on your Mac.

Using Your New RAID 1 Mirror Set

OS X treats RAID sets created with Disk Utility as standard hard drive volumes. As a result, you can use them as startup volumes, data volumes, or backup volumes.

Now that you have finished creating the RAID 1 mirror set, here are some tips for its use.

Hot Spares

You can add additional volumes to a RAID 1 mirror at any time, even after the RAID array is created. Drives added after a RAID array is created are known as hot spares. The RAID array doesn't use hot spares unless an active member of the set fails.

At that point, the RAID array automatically uses a hot spare as a replacement for the failed hard drive and automatically starts a rebuilding process to convert the hot spare to an active member of the array. When you add a hot spare, the hard drive must be equal to or larger than the smallest member of the RAID 1 mirror set.

Rebuilding

Rebuilding can occur any time a RAID 1 mirror set drive becomes out of sync—the data on a drive doesn't match other members of the set. When this occurs, the rebuilding process begins automatically, assuming you selected the automatic rebuild option during the RAID 1 mirror set creation process. During the rebuilding process, the out-of-sync disk has data restored to it from the remaining members of the set.

The rebuilding process takes time. While you can continue to use your Mac as usual during the rebuild, you should not sleep or shut down the Mac during the process.

Rebuilding can occur for reasons beyond a hard drive failure. Some common events that trigger a rebuild are an OS X crash, a power failure, or improperly turning off the Mac.