What a disk image is and why you'd make one

A disk image is a single file that contains a complete copy of an entire disk or partition — in this case, your Mac's operating system and everything on it. When you create a disk image from your Mac installation, you're making a compressed, portable backup that you can store, share, or use to restore your system later.

The most common reason to create a disk image is to have a bootable backup you can use if your Mac won't start up. Another reason is to create an installer you can use on multiple machines without downloading macOS again. You might also create one to archive your system at a particular point in time, or to move your entire setup to a new Mac.

macOS includes the tools to do this built in — you don't need third-party software, though some people prefer tools like Carbon Copy Cloner or SuperDuper for automated backups.

Key Takeaways

  • The Disk Utility app, built into macOS, can create a disk image from your startup drive or from the macOS installer files.
  • A bootable disk image requires at least as much free space as the data you're copying, plus room for compression.
  • Creating an image from your entire drive takes hours depending on size; creating one from installer files takes minutes.
  • Once created, you can mount the disk image like a regular drive, or use it to restore your Mac using Recovery Mode.

Creating a disk image from your startup drive using Disk Utility

Open Disk Utility (in Applications > Utilities, or search for it with Spotlight). In the menu bar, click File, then click New Image, then click Image from Folder. Select your startup drive (usually called "Macintosh HD" or similar) and click Image.

A dialog box will appear asking where to save the image and what to name it. Choose a location with enough free space — the image file will be roughly the same size as your drive, though compression can reduce it. Give it a descriptive name like "MacBackup-2024" so you know what it contains and when you made it.

Disk Utility will ask you to choose an encryption option. If you want the image encrypted (recommended if it contains personal data), select AES-256 and create a password. If you don't need encryption, select None. Then click Save. The process will begin, and a progress bar will show you how much time remains. Depending on your drive size, this can take several hours.

Creating a disk image from macOS installer files

If you've downloaded macOS from the App Store but haven't installed it yet, you can create a disk image from those installer files instead of from your entire drive. This is much faster and creates a smaller file you can use to install macOS on other Macs.

Open Disk Utility and click File > New Image > Blank Image. Set the size to at least 14 GB (for recent versions of macOS; older versions are smaller). Name it something like "macOS-Sonoma-Installer". Under Image Format, select "read/write" so you can add files to it. Click Create.

Once the blank image is created, it will mount on your desktop. Open Finder and navigate to Applications. Find the macOS installer (it will be named something like "Install macOS Sonoma.app"). Drag it into the mounted disk image. Once it's copied, eject the image. Then open Disk Utility again, select the image in the sidebar, and click File > Convert. Choose "read-only" format and save it with a new name. This makes the image smaller and prevents accidental changes.

Using Disk Utility to create an image from an external drive

You can also create a disk image from an external drive — useful if you're backing up a second drive or preparing to migrate data. Connect the external drive to your Mac and make sure it appears in Disk Utility's sidebar.

Click File > New Image > Image from Folder. Select the external drive and click Image. Follow the same steps as creating an image from your startup drive: choose a save location with enough free space, name the image, decide on encryption, and let it run. The time required depends on how much data is on the external drive.

Restoring your Mac from a disk image

Once you have a disk image, you can restore it to your Mac or to another drive. Connect the target drive (or use your startup drive if you're restoring in Recovery Mode). Open Disk Utility, click File > Restore, and select the disk image as the source. Select the destination drive and click Restore. Your Mac will warn you that this will erase the destination drive — this is normal and expected.

If you're restoring to your startup drive, you'll need to restart in Recovery Mode first. Restart your Mac and hold Command + R until you see the Recovery Mode screen. Then open Disk Utility from the Utilities menu and follow the same restore steps.

Storing and managing your disk images

Disk images can be large files, so think about where you'll keep them. External hard drives are common — a 2 TB or 4 TB drive can hold multiple backups. Cloud storage like iCloud Drive or Google Drive works for smaller images, though uploading a 100 GB image will take a long time on most home internet connections.

Label your images clearly with the date and what they contain. If you create regular backups, consider keeping at least two versions — one recent and one from a few months ago — so you have options if something goes wrong between backups.

Check your images occasionally by mounting them (double-click the .dmg file) and confirming the contents are there. A disk image that's corrupted is useless when you actually need it.

Frequently Asked Questions

How much disk space do I need to create an image?

You need free space equal to the size of what you're copying, plus extra for compression overhead. If your drive is 500 GB and mostly full, you'll need at least 550 GB of free space on the destination drive. If the destination doesn't have enough space, Disk Utility will tell you before it starts.

Can I create a disk image while my Mac is running normally?

Yes. Creating an image from your startup drive while you're using your Mac works fine, though it will be slower because your drive is also handling your regular work. The image will include everything on your drive at the moment you started the process.

What's the difference between read-write and read-only disk images?

A read-write image lets you add or change files after you create it. A read-only image is locked — you can mount it and read from it, but you can't modify it. Read-only images are smaller and safer for backups because you can't accidentally change them.

Can I create a bootable disk image on an external drive?

Yes. Create the image on an external drive using the same steps. To boot from it, restart your Mac and hold Option until you see the startup disk picker, then select the external drive. This is useful if your internal drive fails and you need to start your Mac from a backup.

How long does it take to create a disk image?

Creating an image from your entire drive typically takes 1 to 4 hours depending on drive size and how full it is. Creating an image from installer files takes 10 to 30 minutes. Speed also depends on the destination drive — external USB 2 drives are slower than USB 3 or Thunderbolt drives.