Working with Mac OS

How to install macOS on an external hard drive

The first thing you should know is that it is possible to install macOS on an external hard drive. There can be many reasons why you want to have the macOS operating system on an external disk: you want to only try a beta version and not lose the original or not be sure you want to install the new version.

Installing the Mac operating system on an external drive is the safest way to view and test a new operating system without having to change anything on your Mac. It will leave your current configuration intact and allow you to test a beta version or another operating system.

You may also want to know how to install macOS on a PC.

This is what you need to install macOS on an external hard drive

You will be interested in using a fast drive, it is worth investing in a USB-C or Thunderbolt 3 driveas this will give you a faster connection. If not, at least make sure you opt for storage flash.

That drive will need at least 12GB of space, but more is recommended, in fact, when we start running Mojave beta on a pen-drive of 32GB we realized that it was not enough once we opened iCloud Desktop and other functions that meant that our data began to be downloaded.

If you want to use the external install as if you were running it on your Mac, we suggest you get the largest USB C / Thunderbolt 3 drive you can afford.

Step 1: Prepare for installation

Follow these instructions to prepare your external drive or hard drive:

  1. Connect the external hard drive to your Mac.
  2. Start ‘Disk Utility’ (press ‘Cmd + space bar’ and type ‘Disk Utility’).
  3. Before this next step – if you are running High Sierra or later, you will have to click on the ‘View’ drop-down menu below the ‘Close’, ‘Minimize’ buttons. Select ‘Show all devices’ from the options. You will now see the external root drive in addition to the volume underneath.
  4. Select the unit in the sidebar.
  5. Click ‘Delete’.
  6. Choose ‘Mac OS Extended’ (Journaled) as the format.
  7. Select ‘GUID Partition Map’ as the schema.
  8. Give your drive a name like “macOS Catalina” or “USB.”
  9. Click ‘Delete’.
  10. Wait while disk utility creates the partition and configures the drive (this may take a few minutes).
  11. Then click on ‘Done’.


Step 2: Get the MacOS installation files

This process will depend on whether you want to run the beta version or a full version of macOS, or even an older Mac OS X.

If what you are looking for is a beta version, we have a complete guide to get it macOS beta here.

To download the beta version of Catalina, for example, you will have to do the following:

  1. Sign up as a developer or for the Public Beta program.
  2. Once you have completed the previous step, you will be able to download the Public Beta Access Utility (or Beta Access Utility for Developers) from the Apple website.
  3. The Beta Access Utility DMG file will eventually appear in your Downloads folder. Once it arrives, open it up and click on ‘macOSPublicBetaAccessUtility.pkg’ to install it (it’s 215k).
  4. You may need to enroll your Mac. To check if your Mac is enrolled, open ‘System Preferences> Software Updates’ (in older versions of MacOS, App Store). If your Mac is enrolled, you will see that “Your computer is configured to receive updates from software beta”.


  1. Once the Beta Access Utility is installed, the ‘Software Update’ tool will open and begin downloading the beta version. (If you are on an older version of macOS, you will be redirected to a Catalina page in the Mac App Store – you will not see it if you are not registered and running the login utility). Click ‘Download’ and wait while it downloads to your Mac. The download may take a while as the file is over 5GB, but you should be able to continue working while the download is in progress.


  1. Once the download is complete, your Mac will launch it. WARNING: You are not planning to install it on your Mac, so DO NOT CLICK CONTINUE. Instead, exit the installer.
  1. You will find the ‘Install macOS Catalina Beta’ installer in your Applications folder.

Step 3: Install macOS on the external drive

Now, to install the beta or alternative version of the Mac operating system on your external hard drive:

  1. Open a search window, go to ‘Applications’ and find the Catalina Beta installer (or whatever version of macOS you are running).
  2. Click on the installer.
  3. In the first window, to configure the installation of macOS Catalina Beta (or whatever) click on ‘Continue’.
  4. Accept the license agreement of the software.
  5. Next, and this is important, click on ‘Show all disks’. Don’t click ‘Start’ as you don’t want to install on the main unit.
  6. Click on the external drive.


  1. Click ‘Install’.

Wait while the macOS version installs on the drive. This process can take a long time, so don’t despair.

These steps will not have worked if you did not configure the external drive as a GUID partition.

Step 4: Run macOS on the external drive

There are two ways to run the macOS version from your external drive.

Either:

  1. Open ‘System Preferences> Startup Disk’.
  2. Select the external drive you want to use to start your Mac and click ‘Restart’.

OR:

  1. Hold down the ‘Option / Alt’ keys during startup.
  2. Choose the drive you want to use to start up your Mac.

For more tips on installing macOS or Mac OS X, read How to repair your Mac when updating macOS and How to install older versions of OS X on a Mac.

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