Dual Boot OS X 10.7 & OS X 10.8

OS X Mountain Lion is Apple’s latest Mac operating system that is complete with a bunch of new features that iOS users will find familiar. It’s looking like a great addition to the Mac OS family, but for the time being it’s still in Developer Preview, has a fair amount of bugs, and isn’t quite ready for prime time. For those who want to explore and develop for OS X Mountain Lion without losing their primary – and stable – OS X Lion installation, the best thing to do is create a dual boot set up. This lets you have both Mac OS X 10.7 and OS X 10.8 on the same Mac, which you can easily switch between with a reboot.

Before beginning, do the following:
  • Check system requirements to insure OS X Mountain Lion compatibility
  • Download OS X Mountain Lion from the App Store
  • Back up the Mac within OS X Lion using Time Machine
If you already made an OS X Mountain Lion boot installer, you can use that, or you can partition directly from Disk Utility in Lion. It shouldn’t matter although OS X Lion can be pickier with partitioning the boot drive than prior versions of Mac OS X. After you have backed up your Mac, you can proceed.

Set Up Dual Boot for OS X Lion & Install OS X Mountain Lion
We’ll walk through partitioning, installing, and booting either version of Mac OS X:
  • Open Disk Utility, click on hard drive and then click on “partition”
  • Click the + icon to add a new partition, make it at least 14GB and name it something obvious like “Mountain Lion”, then click “Apply”
  • Confirm the creation of the partition (see note below if you have problems here)
  • Launch the OS X Mountain Lion Preview installer (or mount the InstallESD.dmg file) and open the “Install OS X Mountain Lion Preview 1.app” to begin installation
  • Click through to install, and select the newly made partition “Mountain Lion”
  • Let installation commence, the Mac will reboot and installing OS X Mountain Lion will begin
You’re done! After installation is complete, you will be greeted by the familiar OS X welcome and set up screen.

Choosing which OS X version to boot
Hold down Option during reboot to bring up the boot loader. You’ll find four drives; one for OS X 10.7, one for OS X 10.8, and a recovery partition for each OS X version. This is why it was important to name the OS X Mountain Lion partition something obvious.

Select “Mountain Lion” and boot as usual. Boot disk settings can also be adjusted through System Preferences’s Startup Disk panel at any point in either Mac OS X 10.7 or OS X 10.8.

Competing Recovery Partitions
Having two active recovery partitions can cause issues if you attempt to restore from one or the other, and it’s recommended to not use either to restore the OS in the current dual boot setup. If you are going to remove either the OS X Lion or OS X Mountain Lion partitions, don’t forget to remove the Recovery partition accompanying that version of Mac OS X. If not, you could inadvertently restore the wrong OS, or encounter boot problems if you delete the wrong OS and find the recovery partition to be incompatible with the remaining version of OS X.

Note about partitioning in OS X 10.7
OS X Lion is pickier than past versions of Mac OS X when partitioning the boot drive. If you run into problems partitioning the active boot drive, reboot into Recovery Mode (Command+R at boot) and use Disk Utility from there to create the partition, then reboot again into OS X Lion and continue with installation.