Concerned about updating your Mac to the latest version of macOS? Perhaps you aren't sure about running macOS Monterey quite yet, or maybe you want to test the beta but don't want to risk messing up your system. You might be concerned about incompatible software or you might just be thinking that you might hate it and not be able to go back to the macOS you are familiar with. 

Luckily you don't have to install the OS update on your Mac at all - you can install it on an external 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 Monterey or try out the beta of the next version of macOS.

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 on an Intel-powered Mac, or pressing and holding the on switch on an M1 Mac.

What you need

If you want to run macOS on an external drive you will need a few things including a drive with a good amount of storage available.

Make sure that you opt for flash storage because Apple's APFS (Apple file system) is optimized for SSD and the computer will be sluggish if you attempt to boot from a hard drive. It's also worth investing in a USB C or Thunderbolt 3 drive as this will offer the fastest connection.

You can get hold of a USB flash stick pretty cheaply, such as this San Disk USB Type-C drive with 256GB for £30.93 (RRP £59.99). It's a bit harder to find a Thunderbolt powered SSD, and unfortunately they are a lot more expensive. One example is the Samsung X5 Thunderbolt 3 1TB SSD, currently £273.98 from Amazon (RRP £344.99).

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

Troubleshooting problems with the M1 Mac

Before we move on to how you can create this external installation of macOS we will mention some issues people had when trying to create such an installation for an M1 Mac.

After the first M1 Macs launched in 2020 it emerged that there was a problem with the USB interface. As a result some users encountered problems trying to install macOS on and booting from SSDs connected via USB. This issue was quickly addressed in Big Sur 11.0.1, improved further in 11.1 and refined again in 11.2. The problem has now been addressed by Apple's developers, but if you find it affects you we suggest that you do the following:

  • Try a different port. For some it has worked to install on a disk connected via USB-A connector, but not USB-C, while the situation has been vice versa for others.
  • Another solution is to use a drive that uses Thunderbolt rather than USB (the port looks the same, but the standard is different). It should be connected directly to the computer, not via a dock. That drive will need a good amount of space (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).

Another issue worth flagging is that if you were hoping to use this boot drive to run older versions of macOS on M1 Macs sadly it isn't possible as the M1 Mac can only run the ARM version of Big Sur.

Install MacOS on a external drive

Step 1: Prepare the drive for installation

Follow these instructions to prepare your external drive - note that there may be some slight changes to the process if you wish to install the macOS Monterey or the latest beta.

  1. Connect the external 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.

Whether you want to get the macOS Monterey or the latest beta installation files the process to be similar to that detailed below. When Monterey becomes available to download on 25 October 2021 you should be able to get the installation files via Software Update or the Mac App Store.

Here's how to get the macOS Big Sur or Catalina installer:

  1. To download the macOS Big Sur installer click here to go to the Big Sur page on the Mac App Store. To download the macOS Catalina installer click on this link to the Mac App Store.
  2. Click on Get to download the installer.
  3. Software Update will search and then, if you are already running Big Sur (or Catalina), if will ask you to confirm that you do want to download macOS Big Sur (or 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, you will need to sign up for the beta on your Mac and download the beta files. Once you have the files you'll want to install them on the external drive rather than your Mac, so be careful. We have a complete guide to getting the macOS beta here. The beta files are about 12GB.

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 or Big Sur 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 not). It will invite you to set up the installation of macOS Monterey (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 on an Intel-powered Mac, or press and hold the on button on an M1 Mac until you see the start up options.
  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.