Dual Boot: Run macOS Catalina with Mojave on the same Mac

If you’re excited at the launch of macOS Catalina and want to try out all the new features for yourself, you can download the public beta from July and give it a go. However, as it’s beta software, it comes with a health warning and you should take precautions when testing it. One way you can do that is to create a dual boot system so you can still boot into Mojave, or whatever macOS you’re currently running, when you need to. Here’s how to dual boot macOS Mojave and macOS Catalina.

Clean up your drive beforehand

Once installed, macOS Catalina will have access to all your files and folders, so back up your Mac just in case it damages them. You can use Time Machine or any other backup tool. Just make sure you have a backup of your entire Mac before you download macOS Catalina. You should also consider turning on FileVault. It will slow your Mac down, but will force Catalina to ask for permission before it access encrypted files and folders.

The other thing you should do is clear out the junk on your Mac. We’re going to install Catalina on your main startup disk (yes, you don’t need an external disk!), so the more space you have available, the better. It’s not often that I recommend specific applications, but for this purpose, I’m a big fan of CleanMyMac X. It makes it really easy to identify what’s taking up all your disk space, thanks to its System Junk tool. And it makes it easy to remove junk files, too. You can either let it scan your whole Mac, or focus on specifics like iTunes, Photos cache files, Launch agents, or large and old files you no longer need to access regularly. You can download it here, so I recommend giving it a try, you might find that it frees up tens of gigabytes of space.

CleanMyMac X - User cache files in System Junk module

What you’ll need for macOS Catalina

  1. Access to macOS Catalina. It’s only available to Apple developers until July. Then there will be a public beta available. In October, the full version will be available on the Mac App Store.
  2. A Mac that’s capable of running Catalina. Here’s the list of compatible Macs:

MacBook 2015 and later

MacBook Air 2012 and later

MacBook Pro 2012 and later

IMac 2012 and later

IMac Pro 2017 and later

Mac Pro 2013 and later

3. An APFS-formatted startup disk. If your Mac is running High Sierra or Mojave, its startup disk is probably formatted as APFS. The exception is hard disks or Fusion drives running High Sierra. You can check the format of your startup drive by clicking Launchpad > Disk Utility.

If your start-up disk is not formatted as APFS, your best option is to install Catalina on external drive. In this case, instead of using Disk Utility to create another volume on your startup disk as shown below, you should grab a USB stick or external hard drive with at least 16GB capacity and use Disk Utility to erase it and reformat it. Then, when you install Catalina, below, choose it, rather than the second volume, as the location for the install.

Download and install macOS Catalina

Depending on when you read this, macOS Catalina will either be in developer beta, public beta, or full release. If it’s still in developer beta, you’ll need to wait until the full public beta is available in July, unless you’re a registered Apple developer.

Before you download macOS Catalina, you’ll need to create a volume on which it can be installed. If you’re running Mojave, your startup disk will be formatted as APFS. This makes it easy, because APFS doesn’t require you to create a new partition. It uses containers, each of which can host multiple volumes. Here’s how to create one for Catalina.

  1. Go to Applications>Utilities and double-click Disk Utility to launch it.
  2. Select your startup volume in the left sidebar.
  3. Click on the Edit menu and choose Add APFS Volume.
  4. When “Add APFS Volume to container” appears, give your new volume a name, say “Catalina”, and choose APFS as the format.
  5. You can ignore the section for reserving space for the new volume, it will be given what it needs as it shares space with your startup volume.

You’re now ready to download and install Catalina.

  1. Click here to begin downloading the public beta.
  2. When the download starts it will prompt you to choose a location for the installation. Choose the volume you just created.
  3. Click process to install macOS Catalina

Your Mac will restart several times during the installation process. Once it’s finished, you will be able to dual boot into wither macOS Catalina or macOS Mojave.

How to dual boot macOS Mojave and macOS Catalina beta

Once you’ve installed macOS Catalina, you have two options for choosing which OS to boot into.

  1. Hold down the Option key when you restart or boot your Mac, then choose macOS Catalina or Mojave.
  2. Before you shutdown or restart, go to the Apple menu and choose System Preferences. Then choose Startup Disk and select the one you want to boot into next time.

My apps don’t work on macOS Catalina

First thing people notice after they run a new OS is some of their apps stopped working. Annoying as it is, not all developers have yet optimized their software for Catalina. A large portion of your applications may still be 32-bit, which macOS Catalina won’t support.

There’s a quick 5 second way to check which of your apps are 32-bit.

I do it using the Uninstaller tool in CleanMyMac X. You can download its free version here.

  1. Launch the program and click the Uninstaller tab
  2. Now, select 32-bit in the sidebar
All apps selected in the Uninstaller module of CMMX

From there you can find your long-forgotten apps, delete them. And, most importantly, update them to the latest versions using the Updater tool (the one below Uninstaller).

Dual-booting macOS Catalina and MacOS Mojave is a great way to try out Apple’s new operating system without having to use it every day. You should never use a beta version of an OS for day to day work, so it avoids that problem too. If your startup disk is formatted as APFS, you can even have both operating systems installed on the same disk. Just remember to backup your Mac before you start, and to use CleanMyMac X to clear out junk files.

Laptop with CleanMyMac
CleanMyMac X

Your Mac. As good as new.