Use Disk Utility to Clone a Mac's Drive

Disk Utility's Restore Function Lets you Create a Bootable Clone

Disk Utility Restore (clone) function
You can use Disk Utilities Restore function to create a clone. Screen shot courtesy of Coyote Moon, Inc.

With OS X El Capitan and later versions of the Mac OS, Apple changed the process for using Disk Utility to clone a Mac’s drive. While it's still possible to create an exact copy (a clone) of any drive connected directly to your Mac, the changes made to Disk Utility means there are extra steps involved if you wish to use Disk Utility’s Restore function to clone your startup drive.

But don't let the idea of extra steps get in the way, the process is still pretty simple and the added steps actually help ensure a more accurate clone of the startup drive.

Disk Utility’s Copy Function

Disk Utility has always been able to create clones, although the app refers to the process as Restore, as in restoring data from a source drive to a target drive. To be clear, the restore function isn't limited to drives; it will actually work with just about any storage device that can be mounted by your Mac, including disk images, hard drives, SSDs, and USB flash drives.

How Restore Works

The Restore function in Disk Utility makes use of a block copy function that can speed up the copy process. It also makes an almost exact copy of the source device. When I say "almost exact" I don't mean to imply that useful data may get left behind, because that's not the case. What it means is that a block copy copies everything in a data block from one device to the other. The results are almost an exact copy of the original. A file copy, on the other hand, copies data file by file, and while the file data remains the same, the location of the file on the source and destination devices will likely be very different.

Using a block copy is faster, but it does have some limits that affect when it can be used, the most important being that copying block by block requires that both the source and destination devices be first unmounted from your Mac. This ensures that block data doesn't change during the copy process.

Don’t worry, though; you don’t have to do the unmounting. Disk Utility’s Restore function takes care of that for you. But it does mean that neither the source nor the destination can be in use when you use the Restore capabilities.

The practical limitation is that you can’t use the Restore function on the current startup drive, or any drive that has files in use. If you need to clone your startup drive, you can make use of either your Mac's Recovery HD volume, or any drive that has a bootable copy of OS X installed. We'll provide information about how to use the Recovery HD Volume to clone your startup drive, but first, we'll look at the steps in cloning a non-startup drive attached to your Mac.

Restore a Non-Startup Volume

  1. Launch Disk Utility, located at /Applications/Utilities.
  2. The Disk Utility app will open, displaying a single window divided into three spaces: a toolbar, a sidebar showing currently mounted drives and volumes, and an info pane, showing information about the currently selected device in the sidebar. If the Disk Utility app looks different then this description you may be using an older version of the Mac OS. You can find instructions cloning a drive using an earlier version of Disk Utility in the guide: Back Aup Your Startup Disk Using Disk Utility.
  1. In the sidebar, select the volume to which you want to copy/clone data. The volume you select will be the destination drive for the Restore operation.
  2. Select Restore from Disk Utility’s Edit menu.
  3. A sheet will drop down, asking you to select from a drop-down menu the source device to use for the Restore process. The sheet will also warn you that the volume you selected as the destination will be erased, and its data will be replaced with data from the source volume.
  4. Use the drop-down menu next to the "Restore from" text to select a source volume, and then click the Restore button.
  5. The Restore process will begin. A new drop-down sheet will display a status bar indicating how far along in the Restore process you are. You can also see detailed information by clicking the Show Details disclosure triangle.
  1. Once the Restore process is complete, the drop-down sheet’s Done button will become available. Click Done to close the Restore sheet.

Restore Using a Startup Drive

When you use the Restore function, both the destination and the source must be able to be unmounted. This means you can’t be booted to your normal startup drive. Instead, you can start your Mac from another volume that contains a bootable version of the Mac OS. This can be any volume attached to your Mac, including a USB flash drive, an external, or in the example we will use, the Recovery HD volume.

A complete step-by-step guide is available in Use the Recovery HD Volume to Reinstall OS X or Troubleshoot Mac Problems.

Once you've booted from the Recovery Volume and used the step-by-step guide to launch Disk Utility, return here and use the Restore a Non-Startup Volume guide, above, starting at step two.

Why Use Disk Utility’s Restore Function?

You may have noticed over the years that I've recommended cloning apps, such as Carbon Copy Cloner and SuperDuper, for creating bootable clones as part of a backup system.

So, if I think cloning apps are better, why use Disk Utility instead? The reasons can be many, not the least of which are the simple facts that Disk Utility is free, and included with every copy of the Mac OS. And while the various cloning apps have a lot more features, if you don’t have access to the third-party apps, using Disk Utility will create a perfectly usable clone, although it may require a few more steps and lacks some nice features, such as automation and scheduling.

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