We’re going to talk about installing a version of your Mac’s operating system (OS), known as “macOS” or “OS X,” on your Mac, older than the one you’re currently running, on a partition of your primary hard drive or on an external hard drive. You may find that your current instance of OS X is too unstable for normal day-to-day usage or more heavy-duty tasks like development. Remember all the problems people had when they upgraded to OS X 10.13, also known as “High Sierra?” Oy, vey. You might have been like “Get me the heck outta Dodge!” You wanted or needed to get back to a stable OS, like Sierra (OS X 10.12) or El Capitan (OS X 10.11). For developers, you may have to install an older version of Xcode not supported by your latest OS. For Cocoa/macOS developers, you may need to make absolutely sure that your desktop apps are backward compatible, and the only way to do that for sure is to install and run your apps on older versions of macOS. I will show you, step by step, how to get a valid copy of an older version of macOS, make a bootable installer disk, and install the old OS.

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Note to iOS developers

As iOS developers, you’re probably mostly concentrated on using Xcode and perhaps a few supplementary Mac apps, like Safari, TextEdit, or Pages, to develop apps for iPhone (iOS), iPad (iOS), Apple Watch (watchOS), and Apple TV (tvOS). You probably heavily use Git, Xcode command-line tools, or development tools like CocoaPods. If you’ve been developing for more than even a year, then you know that Apple pushes out a new OS about once a year and also pushes out a new Xcode version once a year. It behooves you to know something about OS X. So while this tutorial is of course open to anyone who needs to roll back their OS, my core audience of iOS developers should give it a read and consider practicing developing some macOS apps. There are millions of people out there with Macs. That’s a big potential target audience for whom you can build apps.

Obeying the rules

I’m pretty sure that according to Apple’s rules, you can only run one version of OS X on one Mac at a time. Be sure to consult Apple’s End User License Agreement and its Terms and Conditions before proceeding. As we’ll see in a bit, they do allow you to download old versions of OS X that you previously purchased and/or downloaded through the Mac App Store using a valid Apple ID.

Finding an old version of OS X

Only you can decide which old version of OS X you want to install, but you can’t just grab any version from any time period and install it on any Mac.

Hardware requirements
You need to know if your Mac’s hardware is compatible with the old version of OS X you’ve chosen to install. Here’s a hardware compatibility list for OS X versions 10.6 (Snow Leopard), 10.7 (Lion), 10.8 (Mountain Lion), 10.9 (Mavericks), 10.10 (Yosemite), 10.11 (El Capitan), and 10.12 (Sierra). Here are the requirements for the latest OS X 10.13, High Sierra.

Finding and downloading an old macOS version
You can only get old versions of OS X that you previously downloaded/installed on one of your Macs using your valid Apple ID. Go to the App Store app on your Mac and then select the Purchased tab. Look for the version you want — and one that’s in the list (obviously). Using my Apple ID and browsing my Purchased history, I’m going to get OS X Yosemite by clicking the DOWNLOAD link as show here:

You’ll get a warning that “A newer version of OS X is already installed on this computer. Are you sure you want to download OS X 10.10?” Click Continue as shown:

This will start the download of Install OS X Yosemite.app into your /Applications folder. This file is 5.72 GB, so expect the download to take awhile.

Preparing to install the old macOS version

If you double-click on Install OS X Yosemite.app, you’ll get the warning, “This copy of the ‘Install OS X’ application is too old to be opened on this version of OS X.” Here’s what you’ll see:

Never fear. We can do this!

Creating “a bootable installer for macOS”
Apple has the answer to how we’ll make use of the installer (OS X Yosemite.app) we downloaded. They have a page entitled “How to create a bootable installer for macOS.”

What we’ll do is follow Apple’s advice and “use an external drive or secondary partition as a startup disk from which to install the Mac operating system.” The best way to create a “startup disk” is to use a USB thumbnail drive with “at least 12GB of available storage.” Everything on your USB drive will be erased, so back it up if you need the files contained on it.

In a nutshell, we’re going to use an Apple command, available only from Terminal, that will turn our USB stick into a bootable drive and copy that OS X installer we downloaded to the USB stick.

When I got that USB stick with at least 12 GB of available space, I inserted it into an available USB port on my Mac. The stick was mounted in my OS X 10.13 file system as “/Volumes/Untitled” and showed up in Finder as “Untitled” as you see here:

Now that I had that USB stick mounted, it was time to continue with Apple’s advice. I made the USB stick into a bootable drive containing all the OS X Yosemite installation files.

I was able to copy the command I needed to do this straight off Apple’s page. Here’s their version of the command:

All those forward slashes are just indicators for the command-line interpreter that a space is being used.

For example, Install OS X Yosemite.app in the /Applications folder becomes “/Applications/Install OS X Yosemite.app”. I’m not going into all the dreary details. If you’re interested in what the createinstallmedia command does, read here.

I opened a Terminal window and pasted my createinstallmedia command to the prompt and pressed the return key.

You’ll get — WOW — animated textual feedback while createinstallmedia prepares your USB stick: