If you're worried about updating your Mac to macOS Catalina (perhaps due to incompatible software), or if you are testing the beta version of macOS Big Sur and don't want to risk messing up your system, you don't have to install the update on your Mac at all - you can install it on an external hard drive instead.

Installing the Mac operating system on an external drive is the safest way to get a good look at a new OS without changing anything on your Mac. It'll leave your current configuration intact and allow you to play around with Catalina or try out the beta of macOS Big Sur.

In this tutorial we explain how to run macOS from an external drive in four simple steps:

  1. Prepare the external drive for the installation using Disk Utility.
  2. Get the macOS install files - we'll show you how.
  3. Install macOS on the external drive.
  4. Run macOS on the external drive by pressing Alt/Option at start up.

What you need

You'll want to use a fast drive if you can - it's worth investing in a USB C or Thunderbolt 3 drive as this will offer the fastest connection. If not, at least make sure that you opt for flash storage.

That drive will need at least 12GB of space, but more is recommended, in fact back when we started running Mojave beta on a 32GB stick we found it wasn't enough once we started using iCloud Desktop and other features that meant that our data started being downloaded. Luckily you can get hold of a USB flash stick pretty cheaply, such as this SanDisk 128GB Flash Drive, which we got on Amazon for £15.89. 

If you want to use the external installation as if you are running it on your Mac we suggest you get the biggest USB C/Thunderbolt 3 drive you can afford.

For buying advice, take a look at our guide to the best external drives for Mac.

Install MacOS on a external drive

Step 1: Prepare the drive for installation

Follow these instructions to prepare your external drive:

  1. Connect the external hard drive to your Mac.
  2. Launch Disk Utility (press Cmd + spacebar and start to type Disk Utility).
  3. Before this next step - if you are running High Sierra or later you will need to click on the View drop down below the close/minimise buttons. Choose Show All Devices from the options.  Now you will see the root drive in addition to the external volume below it. Show all Disk Utility
  4. Select the External drive in the sidebar (the next step won't work if you only select the volume - in the screenshot above you would need to select the SanDisk, not the NO NAME).
  5. Click on Erase (you need to erase the drive so you can reformat it properly).
  6. Give your drive a name such as "macOS Big Sur" or "USB".
  7. Choose Mac OS Extended (Journaled) as the Format.
  8. Choose GUID Partition Map as the Scheme.  Format options Disk Utility
  9. Click on Erase.
  10. Wait while Disk Utility creates the partition and sets up the drive (this can take a few minutes).
  11. Then click Done.

Now you will see that your external storage has been renamed with the name you choose and it will be ready for you to install the macOS on it - but there is something else you need to do first.

Step 2: Get the macOS install files

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

Here's how to get the macOS Catalina installer:

  1. To download the macOS Catalina installer click on this link to be taken to the Catalina page on the Mac App Store.
  2. Click on Get to download the installer.
  3. Software Update will search and then, if you are already running Catalina, if will ask you to confirm that you do want to download macOS Catalina.
  4. Click on Download.

Remember that you just want to download the software, not install it, at this stage.

If it's a full version of an older version of  macOS you are after, we explain how to get the installation files for macOS here.

If you are after a beta of macOS, we have a complete guide to getting the macOS beta here.

Step 3: Install macOS on the external drive

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

  1. Open a finder window, go to Applications and find the Catalina of Big Sur beta installer (or whichever version of macOS you want to run). You can also find it by using Spotlight (press Command + spacebar and start typing the name of the OS)
  2. Click on the installer.
  3. A window should open (check the Dock if now). It will invite you to set up the installation of macOS Catalina (or whatever) click Continue.
  4. Agree to the software licence agreement.
  5. Next, and this is important, click on Show All Disks. Do not click on Home as you don't want to install on your main drive.
  6. Click on the external drive.
  7. Click Install.

Wait while the version of macOS installs on the drive. This process may take a while - if it tells you it will take 10 minutes it's lying.

Note: This will not work if you didn't set your external drive up correctly as a GUID Partition as per the steps above.

Step 4: Run macOS on the external drive

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

Either:

  1. Open System Preferences > Startup Disk.
  2. Select the external drive you wish to use to start up the Mac and click Restart.

Or:

  1. Hold down Option/Alt during startup.
  2. Pick the drive you wish to use to start up the Mac.

Once you have installed the macOS you want on the drive it should be easy to install subsequent updates there via System Preferences.

For more advice about installing macOS or Mac OS X read How to update the Mac operating system and How to install old versions of OS X on a Mac.