Every time a new version of a macOS is announced some are delighted but many who depend on certain software groan knowing a new macOS is going to be a bag of hurt and not play nice with their software. We’ve always worked hard to keep the studio community informed with our upgrade articles that contain a database of compatible software for the most current version of macOS. Here is the Mac Catalina upgrade compatibility article.

However, you may be stuck between a rock and hard place needing two versions of macOS on your studio Mac. Or you may just be curious and want to try and see if the newest version plays nice.

In this article we are going to show you how to install two versions of macOS on one Mac, no reformatting required and all thanks to a handy feature built into modern Macs. It’s easy to do and gives you the flexibility of running a dual boot Mac in minutes.

WARNING: This article is for information only. Always back up your drive before making any changes to a hard drive using either Time Machine or a third-party application like Carbon Copy Cloner.

One more note: At this present time the latest Mac Pro 7,1 does not allow users to roll back from Catalina. We will update this information once someone has figured out how to do it.

How To Create A Dual Boot Mac

Historically to create a dual boot computer required the user to partition a drive, or use two different drives, however with macOS Mojave and above, those with Apple File System (APFS) formatted drives, Apple has done some magic. You can now create dynamic volumes on one drive. What this means is you simply add a second volume and the APFS file system will assign drive space as needed, so you don’t even need to pre-allocate space when making the volume - it’s almost magic. From Apple’s support documentation;

“Apple File System (APFS) allocates disk space on demand. When a single APFS container (partition) has multiple volumes, the container’s free space is shared and can be allocated to any of the individual volumes as needed. Each volume uses only part of the overall container, so the available space is the total size of the container, minus the size of all volumes in the container.”

Even better you don’t have to reformat your hard drive, it’s a couple of clicks, here are the instructions from Apple.

Add an APFS volume

Each volume you add to an APFS container can have a different file system format, if needed.

1. In the Disk Utility app  on your Mac, select an existing APFS volume in the sidebar, then click the Add Volume button  in the toolbar.

2. Enter a name for the new APFS volume. (We named ours Mac Big Sur)

3. Click the Format pop-up menu, then choose an APFS format.

   If you want to encrypt the volume, choose APFS (Encrypted) or APFS (Case-sensitive, Encrypted).

4. If you want to manually manage APFS volume allocation, click Size Options, enter values in the fields, then click OK.

   • Reserve Size: The optional reserve size ensures that the amount of storage remains available for this volume.

   • Quota Size: The optional quota size limits how much storage the volume can allocate.

5. Click Add, then click Done.

Install A Second macOS

Now you have a second volume (which in essence is a second drive) you can install a second macOS on it and have a dual boot machine. If you’ve kept a copy of the old installers then use the one you have, otherwise, you will need to download it, MacWorld has an excellent article on getting hold of old macOS installers.

If you want to go forward or use a beta you can use the upgrade button in System Preferences. However, once the download to the update has completed you need to make sure you select the new volume for installation. This is how;

• Download the upgrade

• Agree to the terms

• Select Show All Disks to switch from your main partition.

• Select the partition you wish to install the new software on. 

• Click Install.

Booting To A Different OS

Now you have a dual boot Mac, booting to a different OS is simple. It can be done in two ways;

1. Go to Startup Disk in Disk Utilities and choose the drive you want to startup from. Then hit restart.

2. Or hold down the Option key on Restart and choose your startup drive from there.

Summary

It’s a simple as that to have a dual boot Mac and is a great way to either have two systems for those who need to jump back and forth between new and legacy software or those who want to test a new macOS before fully committing to it.