[Full Guide] How to Install macOS on an External Drive

Jerome Peng
Last updated: Sep 27, 2023

Looking for instructions on how to install macOS on an external drive? Look no further! In this guide, we will provide you with a step-by-step process to accomplish this.

Installing macOS on an external drive is beneficial, especially if you want to test applications on an older macOS, troubleshoot issues with your current macOS, or experience new features of a new macOS when you don't have enough storage space on your internal hard drive.

Wondering how to do it? Check out this guide where you'll find step-by-step instructions.

[Full Guide] How to Install macOS on an External Drive

How to Install a Newer macOS on Your External Drive

When you want to install a beta version of macOS, it's recommended to install it on an external hard drive so you don't have to worry about anything unexpected happening to your main Mac.

Tools Needed:

  • An external hard drive: at least 20GB (the bigger the better).
  • A full macOS installer.

Step #1. Format Your External Drive

  1. Insert your external drive into your Mac.
  2. Open Disk Utility via Spotlight or Finder.
  3. In the sidebar, select your external hard drive.
  4. Click the Erase button in the toolbar.
  5. If necessary, rename your drive, select APFS as the format and GUID Partition Map as the scheme, and click the Erase button.
Erase External Drive on Mac with Disk Utility

Step #2. Download the macOS Installer

Below is the full installer for recent macOS versions. You can download them from the App Store.

  1. Open the link of the macOS installer you want to download.
  2. Click the Get button to download it. (Your macOS installer will be downloaded to the Applications folder.)
  1. To download the desired macOS installer, use a Mac that is compatible with that macOS.
  2. The macOS installer is approximately 12GB in size. Make sure your Mac has enough storage space. If your storage space is low, it is recommended to use BuhoCleanerto clean up your Mac with one click.

Step #3. Install macOS on Your External Drive

  1. Once the download is complete, the installer will open automatically.
  2. Click Continue and agree to the terms of the software license agreement.
    Install macOS Sonoma
  3. Click Show All Disks, select your external drive, and click Continue.
    Selet Disk to Install macOS Sonoma
  4. Follow the on-screen instructions to install macOS on the external drive. (Please be patient, this process can take up to 1 hour.)
    Start Installing macOS Sonoma
If you have your external drive connected to your Mac, your Mac will start up from the external drive. Otherwise, your Mac will boot from the internal drive. To quickly set up your startup disk, click System Settings > General > Startup Disk, select the system you want to use, and click Restart.

How to Install an Older macOS on Your External Drive

Older macOS installers won't open on newer macOS versions. In this case, you can use a bootable macOS installer to install an older version of macOS on an external drive.

Tools Needed:

  • A USB drive or hard drive to create a bootable macOS installer (at least 14GB).
  • An APFS formatted drive for macOS installation.

Related Article: How to Format an External Hard Drive on Mac

Step #1. Prepare Your External Drive

The first critical step is to create a partition on your external drive in preparation for creating the required bootable installer.

  1. Insert a USB drive or hard drive into your Mac.
  2. Open Disk Utility via Finder or Spotlight.
  3. In the Disk Utility sidebar, select the drive you inserted.
  4. Click the Partition button in the toolbar.
  5. Click the + button below the chart.
  6. Rename the new partition, choose macOS Extended (Journaled) as the format, set its size, and click Apply.
  7. Once the operation is complete, click Done.
Create a New Partition on External Drive
It's recommended that you format your drive as macOS Extended (Journaled), as APFS disks may not be used as bootable installation media.

Step #2. Download the macOS Installer

Use the Apple App Store:

Use the Web Browser:

Once the download is complete, you can find the macOS installer in the Applciations folder.

Step #3. Create Your Bootable Installer

  1. Open Terminal via Finder or Spotlight.
  2. Paste the appropriate command into the Terminal window based on the macOS installer you downloaded.
    • Ventura: sudo /Applications/Install\ macOS\ Ventura.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
    • Monterey: sudo /Applications/Install\ macOS\ Monterey.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
    • Big Sur: sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
    • Catalina: sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
    • Mojave: sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
    • High Sierra: sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Don't forget to replace MyVolume with the name of the new partition you created.
  1. Press Enter to run the command.
  2. When prompted, enter the administrator password and press Enter.
  3. Wait for the creation process to complete. (You'll see a message in the Terminal when it's done.)

Step #4. Install macOS on Your External Drive

Now that you have a bootable installer ready, let's see how to install macOS on your external drive.

  1. Plug the drive containing the bootable installer into your Mac.
  2. Connect an APFS formatted hard drive to your Mac.
  3. Turn on your Mac and hold down the Power button (on Apple Silicon) or the Option (Alt) key (on Intel processor) to access the Startup Options window or the bootable volume screen, respectively.
  4. Select your bootable macOS installer and click Continue.
  5. Select a volume to recover and click Next.
  6. Click Continue, and select the external APFS hard drive you connected.
  7. Follow the on-screen installation instructions to install macOS on your external drive.
You can only install macOS on a hard drive with a GUID Partition Map scheme. You can check the partition scheme of an external disk using Disk Utility. If it has a scheme other than GUID Partition Map, select your drive, click the Erase button in the Disk Utility toolbar, select GUID Partition Map as the scheme, and click Erase.


  1. How Much Space Does It Take to Install macOS on an External Drive?

    The size depends on the version of macOS you are installing. In general, it is recommended to prepare an external drive with a capacity between 20GB and 30GB. This is because the full macOS installer is around 12GB, and you need additional space for the installation. If you plan to install apps and download many files for daily use, it is advisable to keep even more free space.

  2. Can I Install macOS on an External Drive from Windows?

    No, you cannot install macOS on an external drive using a Windows computer. If you want to experience macOS features on a Windows computer, use a virtual machine such as VirtualBox or VMWare.

  3. Can I Install macOS on a USB Drive?

Yes, you can. But, it is more recommended to install macOS on a solid hard drive, as the write and read speed of a USB flash drive is much lower than an SSD.

  1. What Should I Do if I Fail to Install macOS on an External Drive?

    If your installation of macOS on an external drive fails, please check the following and try again:

    • Ensure that your external drive is properly formatted.
    • Verify that your external drive has sufficient storage space.
    • Make sure your external drive is connected to your Mac and receiving power.
    • Double-check that you followed the steps to create a bootable installer accurately.


Following the steps above, you should have successfully installed macOS on your external drive.

It's worth noting that macOS installed on an external drive may not perform as well as when installed on an internal hard drive.

Jerome is a passionate enthusiast of all things Apple. He is expertise in crafting tech-related articles, with a portfolio of more than 100 articles covering various Apple products like the iPhone, iPad, and Mac.