What a disk image is and why you might need one

A disk image is a single file that contains a complete copy of everything on a disk or installation media — in this case, your Mac's operating system installer. When you create a disk image from macOS installation files, you end up with one .dmg or .iso file that you can store, share, or use to reinstall your system later without needing the original installer or an internet connection.

The most common reason to do this is backup: if you ever need to reinstall macOS and your internet is down, or if you want to set up multiple Macs from the same installer without downloading it each time. A disk image also lets you burn the installer to a USB drive or external hard drive more reliably than copying files directly.

Key Takeaways

  • You can create a disk image from the macOS installer app using the Disk Utility program that comes built into every Mac.
  • The installer app itself (found in your Applications folder) is what you convert into an image file, not the Mac's entire hard drive.
  • The resulting .dmg file is usually 10 to 14 gigabytes, so you need external storage or a large USB drive to hold it.
  • Once you have the disk image, you can use it to create bootable installation media or store it as a backup for future reinstalls.

Finding and preparing the macOS installer

Before you can create a disk image, you need the installer app itself. If you downloaded macOS from the App Store, the installer stays in your Applications folder after the read completes. Open Finder, click Applications, and look for an app named something like "Install macOS Sonoma" or "Install macOS Ventura" — the exact name depends on which version you have.

If you don't see an installer app, you can read one directly from the App Store. Search for the macOS version you want, click Get, and let it read. The installer app will appear in Applications once the read finishes. Do not run the installer yet — you only need the app itself to exist in your Applications folder.

Using Disk Utility to create the image

Open Disk Utility, which lives in Applications > Utilities. You can also press Command+Space, type "Disk Utility", and press Enter. Once it opens, go to the menu bar at the top and click File > New > Disk Image from Folder.

A dialog box will appear asking you to choose a folder. Navigate to Applications, find your installer app (for example, "Install macOS Sonoma"), and select it. Click Open. Disk Utility will then ask you where to save the resulting image file and what to name it. Choose a location with enough free space — the image will be 10 to 14 gigabytes depending on the macOS version.

In the dialog box, you can also choose the image format. Select "DVD/CD master" if you plan to burn this to physical media, or "compressed" if you just want to save space. Compressed images take longer to create but use less storage. Click Save and let Disk Utility work — this can take 10 to 30 minutes depending on your Mac's speed and which format you chose.

What happens after the image is created

Once Disk Utility finishes, you'll have a .dmg or .cdr file (depending on the format you chose) sitting in the location you selected. This file is your disk image — a complete, portable copy of the installer. You can move it to an external hard drive, copy it to another Mac, or keep it as a backup.

If you created it in "DVD/CD master" format (.cdr), you can convert it to a standard .dmg file for easier sharing. Right-click the .cdr file, select Rename, and change the extension from .cdr to .dmg. Your Mac will recognize it as a disk image from that point on.

Using the disk image to create bootable installation media

Once you have the disk image, you can use it to create a bootable USB drive or external hard drive that will let you reinstall macOS on any Mac. To do this, you'll need a USB drive or external drive with at least 16 gigabytes of space, and you'll use a tool called createinstallmedia that Apple includes with the installer.

Plug in your USB drive, open Terminal (Applications > Utilities > Terminal), and type the following command, replacing "MyDrive" with the actual name of your USB drive:

sudo /Applications/Install\ macOS\ Sonoma.app/Contents/Resources/createinstallmedia --volume /Volumes/MyDrive

Press Enter, type your Mac password when prompted, and let the process complete. This can take 15 to 45 minutes. When it finishes, you'll have a bootable installer on your USB drive that works independently of the disk image.

Storing and protecting your disk image

Once created, your disk image is just a file — treat it like any other important file. Store it on an external hard drive, a second internal drive, or cloud storage if you have the space. Keep it in a location where you'll remember to find it if you need to reinstall macOS.

If you're storing it long-term, consider keeping a copy in two places. Hard drives fail, and cloud storage accounts can be compromised or deleted. Having a backup of your backup means you're not stuck if one copy becomes unavailable.

Frequently Asked Questions

Can I create a disk image from my entire Mac hard drive instead of just the installer?

Yes, but that's a different process and serves a different purpose. A full hard drive backup is called a Time Machine backup or a clone, and it's used for complete system recovery. A disk image of just the installer is smaller and lets you reinstall a clean copy of macOS on any Mac. For most people, the installer image is what you want.

What's the difference between .dmg and .iso formats?

.dmg is Apple's native disk image format and works seamlessly on any Mac. .iso is a universal format that works on Windows, Linux, and Mac. If you plan to use the image only on Macs, .dmg is fine. If you might need it on other systems, convert to .iso using a tool like Disk Utility or a third-party converter.

Do I need the disk image if I can just read the installer again?

Not strictly, but a disk image is useful if your internet is slow, unreliable, or unavailable when you need to reinstall. It also saves time if you're setting up multiple Macs — you read once and use the image on all of them. For a single Mac with good internet, downloading fresh each time is simpler.

Will the disk image work on older or newer Macs than the one I created it on?

Yes. A disk image of the installer is version-specific (an image of Sonoma installs Sonoma), but it works on any Mac that can run that version, regardless of when the image was created. Check Apple's documentation for which Mac models support each macOS version.

Can I edit or add files to the disk image after I create it?

You can mount the disk image (double-click it) to see what's inside, but you cannot modify the installer files themselves without breaking the installation. If you need to add other files to the image for distribution, create a separate disk image instead of modifying the installer one.