As a developer, of course I always anxiously upgrade to the lastest beta macOS, forgetting that I’ll need to keep the older one around for testing our apps. Then, I need to install an older macOS on a different partition. The challenge is that, if you do the obvious thing and launch the “Install <earlier version>” app, a dialog will appear and tell you This copy of the ‘Install OS X/macOS’ applications is too old to be opened on this version of OS X/macOS – Quit. Arghhh. The following instrucions will work around that.
First of all, understand that it is definitely necessary to have three volumes to do this. A “volume” is a disk “partition”.
Volume N - The volume with the new version of macOS which you are currently running.
Volume T - A “trampoline” volume which will become your bootable installer. This volume must be only big enough to hold the installer. This Volume T will be comnpletely erased during this process. I used a 8 GB thumb drive for Volume T.
Volume D - The destination volume which is where you want the older version of macOS to be installed
Note that these three volume/partitions may or may not be on the same disk.
If you try to use one volume for T and D, everything will proceed OK until it inexplicably tells you that your disk image is damaged and you should redownload from Apple, which will not help.
Part 1 - Prep
• If you are at all short of disk space, switch off Time Machine.
Part 2 - Get the Installer App
• If you want a really old version, you may need to visit developer.apple.com, find the Downloads section, then the macOS version you want, and click the button to view it in the app store.
• “Install” from the Mac App Store. You’re really just downloading it to your /Applications folder. After the download is complete, a dialog will appear indicating that the installer is too old to run on this Mac. Don’t lworry about that.
• Cick the “Quit” button.
Part 3 - Create an Installer Image
• You are going to do the steps in the this Apple article, but read the hints which follow:
Hint 1: The volume on which you are going to create the bootable installer, the “MyVolume” in step 4, is your Volume
Hint 2: In the March 2016 revision of this document, step 4 is kind of upside down. Read it all through. I have submitted document feedback. Maybe they will improve it for a future revision.
Hint 3: The createinstallmedia command will rename your Volume T to Install <whatever>.
Part 4 - Install
• Restart with option key down.
• Select your Volume T, which is now named Install <whatever>.
• When presented with the utility menu, choose option Install OS X/macOS.
• Choose to install on your Volume D.
• Follow other instructions as given by the installer.
Part 5 - Clean up
• Restart into your normal Volume N.
• In your /Applications, delete the “Install <whatever>” app.
* If you’d switched it off, switch Time Machine back on.