The public betas for all of Apple’s latest operating system iterations are finally out (save for watchOS, per usual). We always warn people about installing these on their primary devices, because you don’t want the iPhone you rely on for work and socialising to start freezing up a lot with a buggy version of iOS.
If you’re a Mac user, you have it easy. Instead of replacing your operating system - bad! you can create a brand-new volume on your drive and install macOS Catalina there.
You’ll then be able to select whether you want to boot into your trusty old macOS or the brand-new beta whenever you want, and you can easily remove the Catalina beta from your system when you’re done playing around. Unless you’re hurting for space on your system, there’s no reason why you shouldn’t go this route for the Catalina beta.
I'm mostly talking about the iOS betas here.
NOBODY should ever run Mac betas except Mac-app developers, and even they should always use extra Macs, separate partitions, or external SSDs rather than upgrading their important installations.
— Marco Arment (@marcoarment) June 24, 2019
Apple has a great rundown of everything you need to do to make this happen. Here are the key points:
Back up your system before you do anything (just in case)
Open Disk Utility, select your current volume on the left-most sidebar, and click the + icon under “Volume” on the toolbar to add a new APFS volume.
Make sure you’re enrolled in the Apple Beta Software Program. When you download macOS Catalina and are ready to install it, pick the new volume, not your existing volume, as the target.
Once you’ve installed macOS Catalina, switch between your operating systems by using “Startup Disk” in System Preferences or by holding the Option key as your Mac boots up.
To delete macOS Catalina, all you have to do is delete the APFS volume where it was installed.
Pull up Disk Utility, highlight the volume, and click on the - icon under Volume to get rid of it - easy as that.