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 drive: you only want to 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. This will leave your current configuration intact and allow you to test a beta or other operating system.
You might also want to know how to install macOS on a PC.
Here’s 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 drivebecause it will give you a faster connection. If not, at least make sure you go for storage shine.
This drive will need at least 12GB of space, but more is recommended, in fact, when we start running Mojave beta on a USB key 32 GB, we realized that was not enough once we opened iCloud Desktop and other functions which meant that our data started downloading.
If you want to use the external install just like running it on your Mac, we suggest getting the largest USB C / Thunderbolt 3 drive you can afford.
Step 1: Prepare for installation
Follow these instructions to prepare your external hard drive or hard drive:
- Connect the external hard drive to your Mac.
- Start “Disk Utility” (press “Cmd + space bar” and type “Disk Utility”).
- Before this next step – if you are using High Sierra or later, you will need to click on the “Show” drop-down menu under the “Close”, “Minimize” buttons. Select “Show all devices” from the options. You will now see the external root drive in addition to the volume below.
- Select the unit from the sidebar.
- Click on ‘Remove’.
- Choose “Mac OS Extended” (journaled) as the format.
- Select ‘GUID Partition Map’ as the scheme.
- Give your drive a name such as “macOS Catalina” or “USB”.
- Click on ‘Remove’.
- Wait for Disk Utility to create the partition and configure the drive (this may take a few minutes).
- Then click on ‘Done’.
Step 2: Obtain the macOS installation files
This process will depend on whether you want to run the beta or a full version of macOS, or even an older Mac OS X.
If what you’re looking for is a beta, we’ve got a complete guide to getting it. macOS beta here.
To download the beta version of Catalina, for example, you’ll need to do the following:
- Sign up as a developer or for the public beta program.
- Once you have completed the previous step, you will be able to download the Public Beta Access Utility (or Developer Beta Access Utility) from the Apple website.
- The Beta Access Utility DMG file will eventually appear in your Downloads folder. Once it arrives, open it and click on ‘macOSPublicBetaAccessUtility.pkg’ to install it (it is 215k).
- You may need to register your Mac. To check if your Mac is registered, open “System Preferences> Software Updates” (in older versions of MacOS, App Store). If your Mac is registered, you will see that “Your computer is configured to receive updates from Software
- Once the Beta Access Utility is installed, the “Software Update” tool will open and begin downloading the beta version. (If you’re using an older version of macOS, you’ll be taken to a Catalina page in the Mac App Store – you won’t see it unless you’re registered and running the Login utility). Click “Download” and wait for it to download to your Mac. The download may take a while because the file is over 5 GB, but you should be able to continue working while the download is in progress.
- When 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, quit the installer.
- You can 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 alternate version of Mac operating system on your external hard drive:
- Open a search window, go to “Applications” and search for Catalina Beta installer (or whatever version of macOS you’re using).
- Click on the installer.
- In the first window, to configure the installation of macOS Catalina Beta (or other), click on ‘Continue’.
- Accept the license agreement Software.
- Then, and this is important, click on ‘Show all disks’. Do not click “Start” because you do not want to install on the main unit.
- Click on the external drive.
- Click on ‘Install’.
Wait for the macOS version to install on the drive. This process can take a long time, so don’t despair.
These steps won’t work if you haven’t configured 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.
Any of the two:
- Open ‘System Preferences> Startup Disk’.
- Select the external drive you want to use to start your Mac and click “Restart”.
- Hold down the ‘Option / Alt’ keys while booting.
- Choose the drive you want to use to start your Mac.
For more tips on installing macOS or Mac OS X, read How to fix your Mac while updating macOS and How to install older versions of OS X on a Mac.