macOS Sierra is a fantastic update, but what if you want to keep a copy of OS X El Capitan, too?

If you are preparing to upgrade to macOS Sierra, there is one additional set of steps you should follow. You should download a bootable copy of OS X El Capitan. This allows you to use it if you partition your hard drive for dual-software installation. It also makes it much easier to downgrade to the older format if you decide you don't like macOS Sierra at all.

Before you start

Before you get started, make sure you have a thumb drive with at least 8GB of storage, or a spare external hard drive (one that you don't use with anything else).

You'll also need to download OS X El Capitan from the Mac App Store.

Note: After OS X El Capitan has downloaded, it will automatically begin the installation process. Close the installer instead.

Please, Please, Please, backup your Mac before you do anything.

El Capitan in the Mac App Store

How to format your external drive for Mac

You'll need to start with a clean thumb drive or hard drive in order to get it ready to work as a bootable drive.

  1. Plug the USB end of the drive to a USB port on your Mac.
  2. Click on Finder in your Dock to open a Finder window.
  3. Select Applications from the list on the left side of the window.

    Opening Finder on Mac

  4. Scroll down and double-click on Utilities.
  5. Scroll down and double-click on Disk Utility.

    Selecting Disk Utility on Mac

  6. Select your drive under External.
  7. Click on the Erase tab at the top of the window.

    Erasing a drive on Mac

  8. Name the drive elcap. Or, name it something easy to remember because you will need it when you create a bootable drive.
  9. Select Mac OS Extended (Journaled) from the format list.
  10. Click Erase.
  11. Click Done when the process is complete.
  12. Close the Disk Utility window.

    Confirm erase thumb drive

Your thumb drive or external hard drive is now ready.

How to make your external drive bootable for OS X El Capitan

Important:You will need to be in your Mac's administrator account in order to run the sudo Terminal command

  1. Click on Finder in your Dock to open a Finder window.
  2. Select Applications from the list on the left side of the window.

    Opening Finder on Mac

  3. Scroll down and double-click on Utilities.
  4. Scroll down and double click on Terminal.

    Opening Terminal on Mac

  5. Recall the name of your formatted external drivewhen entering the following text into Terminal. If you did not name it "elcap" you will need to change the command syntax for the pathname where it says: Volumes/elcap.
  6. Enter the following text into Terminal:

    sudo /Applications/Install\ OS\ X\ El\ --volume /Volumes/elcap --applicationpath /Applications/Install\ OS\ X\ El\ --nointeraction

  7. Hit the enter/return key.

  8. Enter your administrator account password. This is the password you use to make changes on your Mac or log in. No text will appear in Terminal when you enter the password.

The process could take a very long time, depending on the drive. When it is done, the Terminal window will report "Done."

The Terminal screen on Mac

How to use OS X El Capitan with a bootable drive

Once macOS Sierra is installed on your external drive, you can boot up any Mac with the drive plugged into it.

  1. Turn off the Mac you want to use OS X El Capitan with the bootable drive.
  2. Connect the drive to your Mac via the USB port.
  3. Turn on your Mac.
  4. Hold down the Option key when you hear the startup chime.
  5. Select the external drive with OS X El Capitan on it from the list of systems to start up your computer.

Choosing the external drive to start macOS Sierra

Do you have any questions about how to create a bootable drive for the macOS Sierra installer? Let us know in the comments.