macOS Disk Utility Can Create Four Popular RAID Arrays

External RAID enclosures
Many external drive enclosures support multiple drives.
01
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macOS Disk Utility Can Create Four Popular RAID Arrays

Mac OS RAID Assistant
The RAID Assistant can be used to create multiple types of RAID arrays. Screen shot courtesy of Coyote Moon, Inc.

macOS Sierra saw the return of RAID support to Apple's Disk Utility, a feature that was removed when OS X El Capitan first came on the scene. With the return of RAID support in Disk Utility, you no longer need to resort to using Terminal to create and administer your RAID systems.

Of course, Apple couldn’t just return RAID support to Disk Utility. It had to change the user interface just enough to ensure that your previous method of working with RAID arrays would be different enough to require learning a few new tricks.

That would be fine if Apple had upgraded the RAID utility to include new capabilities, but as far as I can tell, no updates, either to basic functions or to the RAID driver, are present in the latest version.

RAID 0, 1, 10, and JBOD

Disk Utility can still be used to create and manage the same four RAID versions it has always been capable of working with: RAID 0 (Striped), RAID 1 (Mirrored), RAID 10 (Mirrored set of Striped drives), and JBOD (Just a Bunch Of Disks).

In this guide, we're going to look at using Disk Utility in macOS Sierra and later to create and manage these four popular RAID types. There are, of course, other RAID types you can create, and third-party RAID apps that can manage RAID arrays for you; in some cases, they can even do a better job.

If you need a more advanced RAID utility, I suggest either SoftRAID, or a dedicated hardware RAID system built into an external enclosure.

Why Use RAID?

RAID arrays can solve some interesting problems you may be experiencing with your Mac's current storage system. Perhaps you've been wishing you had faster performance, such as what's available from various SSD offerings, until you realized a 1 TB SSD is a bit beyond your budget. RAID 0 can be used to boost performance, and at a reasonable cost. Using two 500 GB 7200 RPM hard drives in a RAID 0 array can produce speeds approaching those of a mid-range 1 TB SSD with a SATA interface, and do so at a lower price.

Similarly, you can use RAID 1 to increase the reliability of a storage array when your needs demand high reliability.

You can even combine RAID modes to produce a storage array that's fast and retains high reliability.

If you'd like to find out more about creating your own RAID storage solutions to meet your needs, this guide is a very good place to start.

Back Up First

Before we begin the instructions for creating any of the supported RAID levels in Disk Utility, it's important to know that the process of creating a RAID array involves erasing the disks that make up the array. If you have any data on these disks that you need to retain, you must back up the data before proceeding.

If you need assistance with creating a backup, check out the guide:

Mac Backup Software, Hardware, and Guides for Your Mac

If you're ready, let's get started.

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Use macOS Disk Utility to Create a Striped RAID Array

Disk Selection in the Mac OS RAID Assistant
Disk selection is a common process in creating any of the supported RAID types. Screen shot courtesy of Coyote Moon, Inc.

Disk Utility can be used to create and manage a Striped (RAID 0) array that splits data between two or more disks to provide faster access for both data reads from and data writes to the disks.

RAID 0 (Striped) Requirements

Disk Utility requires a minimum of two disks to create a striped array. While there's no requirement for the disks to be the same size or from the same manufacturer, the accepted wisdom is that disks in a striped array should be matched to ensure best performance and reliability.

Striped Array Failure Rate

Additional disks beyond the minimum can be used to increase overall performance, though it comes at the cost of also increasing the overall failure rate of the array. The method to calculate the failure rate of a striped array, assuming all disks in the array are the same, is:

1 - (1 - the published failure rate of a single disk) raised to the number of slices in the array.

A slice is the term commonly used to refer to a single disk within a RAID array. As you can see, the speedier you want to go, the larger the chance of failure you risk. It goes without saying that if you're embarking on creating a striped RAID array, you should have a backup plan in place.

Using Disk Utility to Create a RAID 0 Array

For this example, I'm going to assume you're using two disks to create a fast RAID 0 array.

  1. Launch Disk Utility, located at /Applications/Utilities/.
  2. Make sure the two disks you wish to use in the RAID array are present in the Disk Utility sidebar. They don’t need to be selected at this point; just present, indicating they're successfully mounted on your Mac.
  3. Select RAID Assistant from Disk Utility's File menu.
  4. In the RAID Assistant window, select the Striped (RAID 0) option, and then click the Next button.
  5. The RAID Assistant will display a list of available disks and volumes. Only those disks that meet the requirements for the selected RAID type will be highlighted, allowing you to select them. The usual requirements are that they must be formatted as Mac OS Extended (Journaled), and can't be the current startup drive.
  6. Select at least two disks. It's possible to select individual volumes that a disk may host, but it's considered better practice to use an entire disk in a RAID array. Click the Next button when ready.
  7. Enter a name for the new striped array you're about to create, as well as select a format to be applied to the array. You can also select a "Chunk size." The chunk size should loosely match the predominant size of data your array will be handling. As an example: If the RAID array is being used to speed up the ​​macOS operating system, a chunk size of 32K or 64K would work well, since most system files are generally small in size. If you'll be using the striped array to host your video or multimedia projects, the largest available chunk size may be a better choice.
    Warning: Before you click the Next button, be aware that each disk you have selected to be part of this striped array will be erased and formatted, causing all existing data on the drives to be lost.
  8. Click the Next button when ready.
  9. A pane will drop down, asking you to confirm that you wish to create the RAID 0 array. Click the Create button.

Disk Utility will create your new RAID array. Once the process is complete, the RAID Assistant will display a message that the process was successful, and your new striped array will be mounted on your Mac's desktop.

Deleting a RAID 0 Array

Should you ever decide you no longer need the striped RAID array you created, Disk Utility can remove the array, breaking it back down to the individual disks, which you can then use as you see fit.

  1. Launch Disk Utility.
  2. In the Disk Utility sidebar, select the striped array you wish to remove. The sidebar doesn't show the disk types, so you'll need to select by the disk name. You can confirm it's the correct disk by looking at the Info panel (the lower right-hand panel in the Disk Utility window). The Type should say RAID Set Volume.
  3. Just above the Info panel, there should be a button labeled Delete RAID. If you don’t see the button, you may have the wrong disk selected in the sidebar. Click the Delete RAID button.
  4. A sheet will drop down, asking you to confirm the deletion of the RAID set. Click the Delete button.
  5. A sheet will drop down, showing the progress of deleting the RAID array. Once the process is complete, click the Done button.

Note: Deleting a RAID array may leave some or all of the slices that made up the array in an uninitialized state. It's a good idea to erase and format all the disks that were part of the deleted array.

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Use macOS Disk Utility to Create a Mirrored RAID Array

Mirrored array management option in Disk Utility
Mirrored arrays contain a number of management options including adding and deleting slices. Screen shot courtesy of Coyote Moon, Inc.

RAID Assistant, a component of Disk Utility in macOS, supports multiple RAID arrays. In this section, we're going to look at creating and managing a RAID 1 array, also known as a mirrored array.

Mirrored arrays replicate data across two or more disks, with the main goal of increasing reliability by creating data redundancy, assuring that if a disk in a mirrored array were to fail, data availability would continue without interruption.

RAID 1 (Mirrored) Array Requirements

RAID 1 requires a minimum of two disks to make up the RAID array. Adding more disks to the array increases overall reliability by the power of the number of disks in the array. You can learn more about RAID 1 requirements and how to calculate reliability by reading the guide: RAID 1: Mirroring Hard Drives.

With the requirements out of the way, let's get started creating and managing your mirrored RAID array.

Creating a RAID 1 (Mirrored) Array

Make sure the disks that will make up your mirrored array are attached to your Mac and mounted on the desktop.

  1. Launch Disk Utility, located in the /Applications/Utilities/ folder.
  2. Make sure the disks you intend to use in the mirrored array are listed in Disk Utility's sidebar. The disks don't need to be selected, but they do need to be present in the sidebar.
  3. Select RAID Assistant from Disk Utility's File menu.
  4. In the RAID Assistant window that opens, select Mirrored (RAID 1) from the list of RAID types, then click the Next button.
  5. A list of disks and volumes will be displayed. Select the disk or volume you wish to become part of the mirrored array. You can choose either type, but best practice is to use an entire disk for each RAID slice.
  6. In the Role column of the disk selection window, you can use the dropdown menu to select how the select disk will be used: as a RAID slice or as a Spare. You must have at least two RAID slices; a spare is used if a disk slice fails or is disconnected from the RAID set. When a slice fails or is disconnected, a spare is automatically used in its place, and the RAID array begins the rebuild process to fill the spare with data from the other members of the RAID set.
  7. Make your selections, and click the Next button.
  8. The RAID Assistant will now allow you to set the properties of the mirrored RAID set. This includes giving the RAID set a name, selecting a format type to use, and choosing chunk size. Use 32K or 64K for arrays that will house general data and operating systems; use the larger chunk size for arrays that store images, music, or videos, and the smaller chunk size for arrays used with databases and spreadsheets.
  9. Mirrored RAID sets can also be configured to automatically rebuild the array when a slice fails or is disconnected. Select Automatically Rebuild to ensure optimum data integrity. Be aware that automatic rebuild can cause your Mac to operate slowly while the rebuild is in process.
  10. Make your selections, and click the Next button.
    Warning : You're about to erase and format the disks associated with the RAID array. All data on the disks will be lost. Make sure you have a backup (if needed) before continuing.
  11. A sheet will drop down, asking you to confirm that you wish to create the RAID 1 set. Click the Create button.
  12. The RAID Assistant will display a process bar and status as the array is created. Once complete, click the Done button.

Adding Slices to a Mirrored Array

There may come a time when you wish to add slices to the mirrored RAID array. You may want to do this to increase reliability, or to replace older slices that may be showing issues.

  1. Launch Disk Utility.
  2. In the Disk Utility sidebar, select the RAID 1 (Mirrored) disk. You can check whether you've selected the correct item by examining the Info panel at the bottom of the Disk Utility window; the Type should read: RAID Set Volume.
  3. To add a slice to the RAID 1 array, click the plus (+) sign located just above the Info panel.
  4. From the dropdown menu that appears, select Add Member if the slice you're adding will be actively used within the array, or Add Spare if the new slice's purpose is to serve as a backup to be used if a slice fails or is disconnected from the array.
  5. A sheet will display, listing available disks and volumes that can be added to the mirrored array. Select a disk or volume, and click the Choose button.
    Warning : The disk you're about to add will be erased; make sure you have a backup of any data it may hold.
  6. A sheet will drop down to confirm you're about to add a disk to the RAID set. Click the Add button.
  7. The sheet will display a status bar. Once the disk has been added to the RAID, click the Done button.

Removing a RAID Slice

You can remove a RAID slice from a RAID 1 mirror provided there are more than two slices. You may want to remove a slice to replace it with another, newer disk, or as part of a backup or archiving system. Disks that are removed from a RAID 1 mirror will usually have the data preserved. This allows you to archive the data in another safe location without disturbing the RAID array.

The "usually" disclaimer applies because in order for the data to be retained, the file system on the removed slice needs to be resizable. If the resizing fails, all data on the removed slice will be lost.

  1. Launch Disk Utility.
  2. Select the RAID array from the Disk Utility sidebar.
  3. The Disk Utility window will display all of the slices that make up the mirrored array.
  4. Select the slice you wish to remove, then click the minus (-) button.
  5. A sheet will drop down, asking you to confirm that you wish to remove a slice and that you're aware that the data on the removed slice could be lost. Click the Remove button.
  6. The sheet will display a status bar. Once the removal is complete, click the Done button.

Repairing the RAID 1 Array

It may seem like the Repair function should be similar to Disk Utility's First Aid, just geared to the needs of a RAID 1 mirrored array. But Repair has an entirely different meaning here. Essentially, Repair is used to add a new disk to the RAID set, and force a rebuild of the RAID set to copy the data to the new RAID member.

Once the "repair" process is complete, you should remove the RAID slice that failed and prompted you to run the Repair process.

For all practical purposes, Repair is the same as using the add button (+) and selecting New Member as the type of disk or volume to add.

Since you have to manually remove the bad RAID slice using the minus (-) button when using the Repair feature, I'm going to suggest you just use Add (+) and Remove (-) instead.

Removing a Mirrored RAID Array

You can completely remove a mirrored array, returning each slice that makes up the array back to general use by your Mac.

  1. Launch Disk Utility.
  2. Select the mirrored array in Disk Utility's sidebar. Remember, you can confirm that you selected the correct item by checking the Info panel for the Type being set to: RAID Set Volume.
  3. Just above the Info panel, click the Delete RAID button.
  4. A sheet will drop down, warning you that you're about to delete the RAID Set. Disk Utility will attempt to break the RAID array apart while preserving the data on each RAID slice. There is, however, no guarantee of the data being intact after the deletion of the RAID array, so if you need the data, perform a backup before clicking the Delete button.
  5. The sheet will display a status bar as the RAID is removed; once complete, click the Done button.

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macOS Disk Utility Can Create RAID 01 or RAID 10

RAID 10 infographic
RAID 10 is a compound array made from striping a set of mirrors. Image by JaviMZN

The RAID Assistant that's included with Disk Utility and macOS supports creating compound RAID arrays, that is, arrays that involve the combining of striped and mirrored RAID sets.

The most common compound RAID array is a RAID 10 or RAID 01 array. RAID 10 is the striping (RAID 0) of a pair of RAID 1 mirror sets (a striping of mirrors), while RAID 01 is the mirroring of a pair of RAID 0 striped sets (a mirroring of stripes).

In this example, we're going to create a RAID 10 set using Disk Utility and the RAID Assistant. You can use the same concept for making a RAID 01 array if you wish, though RAID 10 is more commonly used.

RAID 10 is often used when you wish to have the speed of a striped array but don’t wish to be vulnerable to the failure of a single disk, which in a normal striped array would cause you to lose all of your data. By striping a pair of mirrored arrays, you increase reliability while maintaining the improved performance available in a striped array.

Of course, the reliability improvement comes at the cost of doubling the number of disks needed.

RAID 10 Requirements

RAID 10 requires at least four disks, broken into two striped sets of two disks. Best practices say the disks should be from the same manufacturer and be of the same size, although technically, it's not an actual requirement. I do, however, recommend you adhere to best practices.

Creating a RAID 10 Array

  1. Start by using Disk Utility and the RAID Assistant to create a mirrored array made up of two disks. You can find instructions for how to do this on Page 3 of this guide.
  2. With the first mirrored pair created, repeat the process to create a second mirrored pair. For ease of understanding, you may want to give the mirrored arrays names, such as Mirror1 and Mirror2
  3. At this point you have two mirrored arrays, named Mirror1 and Mirror2.
  4. The next step is to create a striped array using Mirror1 and Mirror2 as the slices that make up the RAID 10 array.
  5. You can find instructions for creating striped RAID arrays on Page 2. The important step in the process is to select Mirror1 and Mirror2 as the disks that will make up the striped array.
  6. Once you finish the steps for creating a striped array, you'll have finished creating a compound RAID 10 array.

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Use macOS Disk Utility to Create a JBOD Array of Disks

Adding a disk to a JBOD array in Disk Utility
You can add a disk to an existing JBOD array to increase its size. Screen shot courtesy of Coyote Moon, Inc.

For our final RAID set, we'll show you how to create what's commonly referred to as JBOD (Just a Bunch Of Disks), or as a concatenation of disks. Technically, it isn't a recognized RAID level, as RAID 0 and RAID 1 are. Nevertheless, it's a useful method of using multiple disks to create a single larger volume for storage.

JBOD Requirements

The requirements for creating a JBOD array are quite loose. Disks that make up the array can be from multiple manufacturers, and disk performance doesn't need to be matched.

JBOD arrays provide neither a performance increase nor any kind of reliability increase. Although it may be possible to recover data using data recovery tools, it's likely a single disk failure will lead to lost data. As with all RAID arrays, having a backup plan is a good idea.

Creating a JBOD Array with Disk Utility

Before you begin, make sure the disks you wish to use for the JBOD array are attached to your Mac and mounted on the desktop.

  1. Launch Disk Utility, located at /Applications/Utilities/.
  2. From the Disk Utility File menu, select RAID Assistant.
  3. In the RAID Assistant window, select Concatenated (JBOD), and click the Next button.
  4. In the Disk selection list that appears, pick two or more disks that you wish to use in the JBOD array. You can select an entire disk or a volume on a disk.
  5. Make your selections, and click the Next button.
  6. Enter a name for the JBOD array, a format to use, and a Chunk size. Be aware that chunk size has little meaning in a JBOD array; nevertheless, you can follow Apple's guidelines of selecting larger chunk size for multimedia files, and smaller chunk size for databases and operating systems.
  7. Make your selections, and click the Next button.
  8. You'll be warned that creating the JBOD array will erase all data currently stored on the disks that make up the array. Click the Create button.
  9. The RAID Assistant will create the new JBOD array. Once complete, click the Done button.

Adding Disks to a JBOD Array

If you find yourself running out of space on your JBOD array, you can increase its size by adding disks to the array.

Make sure the disks you wish to add to the existing JBOD array are attached to your Mac and mounted on the desktop.

  1. Launch Disk Utility, if it isn't already open.
  2. In Disk Utility's sidebar, select the JBOD array you created earlier.
  3. To ensure you've selected the correct item, check the Info panel; the Type should read RAID Set Volume.
  4. Click the plus (+) sign located just above the Info panel.
  5. From the list of available disks, choose the disk or volume you wish to add to the JBOD array. Click the Choose button to continue.
  6. A sheet will drop down, warning you that the disk you're adding will be erased, causing all data on the disk to be lost. Click the Add button.
  7. The disk will be added, causing the available storage space on the JBOD array to increase.

Removing a Disk From the JBOD Array

It's possible to remove a disk from a JBOD array, although it's fraught with issues. The disk being removed must be the first disk in the array, and there must be enough free space on the remaining disks to move the data from the disk you're planning to remove to the disks that remain in the array. Resizing the array in this manner also requires that the partition map be recreated. Any failure in any part of the process will cause the process to be aborted and the data in the array to be lost.

It's not a task I suggest undertaking without a current backup.

  1. Launch Disk Utility, and select the JBOD array from the sidebar.
  2. Disk Utility will display a list of the disks that make up the array. Select the disk you wish to remove, and then click the minus (-) button.
  3. You'll be warned about the possible loss of data should the process fail. Click the Remove button to continue.
  4. Once the removal is complete, click the Done button.

Deleting the JBOD Array

You can delete a JBOD array, returning each disk that makes up the JBOD array to general use.

  1. Launch Disk Utility.
  2. Select the JBOD array from the Disk Utility sidebar.
  3. Make sure the Disk Utility Info panel Type reads RAID Set Volume.
  4. Click the Delete button.
  5. A sheet will drop down, warning you that deleting the JBOD array will likely cause all data in the array to be lost. Click the Delete button.
  6. Once the JBOD array is removed, click the Done button.