How to Install an Older macOS

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, 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.  Youre 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.  Dont 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.