Why should I multiboot?

Machines are expensive! If you need to test a product against multiple versions of OS X, it’s VERY costly to purchase an Apple computer per unique configuration.

Multiboot is a powerful tool for both developers & quality assurance testers, and it’s very easy to setup.

How do I get started?

First, you should figure out which versions of OS X you’re going to support.

This tutorial assumes you already have an Apple computer and a valid install of OS X. These instructions were written using 10.10.3 for reference.

Download all OS X installer packages

  1. Open the App Store & browse to the Purchases tab.
  2. Download the versions of OS X you’ll need. If you see an error that prompts Would you like to continue?, press continue.
  3. If your current version of OS X is ahead of the one you just downloaded, you’ll see an error: This copy of “Install OS X” application is too old to be opened on this version of OS X. Dismiss this error by pressing the quit button.

Partition the HDD (Part 1)

While the installer packages are downloading, partition your HDD.

  1. Open the Disk Utility tool.
  2. In the left-hand column, you’ll see a list of hard drives & partitions. Select the APPLE HDD or the top-most mount point in the tree. DO NOT select Macintosh HD.
  3. Browse to the Partition tab. You should see a partition layout. From this list of partitions, select Macintosh HD and then use the to add a partition.
  4. Under Partition Information, choose a name for your new partition. I recommend naming each partition after the OS version it’ll house, e.g. “10.8 Mountain Lion”
  5. Press Apply
  6. Repeat steps 4-5 until you have one partition for each OS you need to install, plus one more partition. So if you need to install 2 additional OS versions, create 3 partitions.
  7. Name the final partition InstallVolume

Mount the Installers (Part 2)

Wait for the OS X installers to finish downloading before proceeding.

  1. Open the Applications folder & find “Install OS X {Version}.app”
  2. Right-click the .app and select **Show Package Contents”
  3. Browse to Contents/SharedSupport and double-click InstallESD.dmg
  4. Open the Terminal and enter the following commands:
  5. defaults write com.apple.finder AppleShowAllFiles YES
  6. killall Finder
  7. When InstallESD.dmg is done mounting, you should see an entry under the Devices menu called OS X Install ESD. Open it.
  8. You should see a greyed-out file called BaseSystem.dmg. Right-click it & Open With DiskImageMounter.app

Write the Installers to InstallVolume (Part 3)

Is your HDD done partitioning yet? Wait for it to finish before proceeding.

  1. In the Disk Utility tool, select InstallVolume from the left-hand list.
  2. Browse to the Restore tab.
  3. Ensure the Source is already InstallVolume!
  4. You should see BaseSystem.dmg in the list of images on the left. Drag BaseSystem.dmg (DO NOT drag OS X Base System) into the Destination field on the right.
  5. Press restore.
  6. When the write is done, the InstallVolume’s name’ will change to OS X Base System

Install the OS X Version (Part 4)

  1. Reboot your Apple computer.
  2. On the grey bootloader screen, press Option. You should see a list of available startup disks.
  3. Select OS X Base System as the startup disk.
  4. Complete the OS X setup instructions. If you’ve gotten this far, you don’t need me to walk you through OS setup.

Repeat Parts 2-4 for each version of OS X you need in your multiboot environment. Continue re-using the “OS X Base System” partition for your install files.

You can set the default startup disk via system preferences