Thu 2/23 Dual Booting Mac OS X

Maybe Apple just released a new developer preview of their desktop operating system you can’t wait to try. Maybe you still need your old operating system for running legacy applications that have not been updated for the latest version of OS X. Maybe you want to give Linux a try without resorting to the performance hit of a virtual machine. Maybe you need to boot into Windows with full hardware compatibility. Whatever the reason you want to make room on your Mac for more than one operating system, a configuration called Dual Booting.

Dual booting is nothing new. Linux users have been doing it for years, and Macintosh users dual booted into Mac OS 9 during the early days of OS X. Bootcamp, software Apple ships with every Mac, guides you through the process of dual booting into Windows. But what if you want to dual boot into two different versions of Mac OS X?

Last week Apple released the first developer preview of Mountain Lion the latest version of OS X. A reckless enthusiast might throw caution to the wind, install 10.8 over their existing operating system, only to complain about the rough edges later. An experienced veteran of beta software knows you can’t easily undo an operating system upgrade and prefers to preserve their current OS while dual booting into 10.8.

Depending on your Mac, dual booting between two versions of OS X can be accomplished in a number of ways.

  • You could carve out a portion of your primary hard drive for an additional OS.
  • You could install a second hard drive to house another version of Mac OS X.
  • You could use a external hard drive with with fast I/O, like FireWire 800 or Thunderbolt, to store Mountain Lion.

Since most Macs don’t have room for two internal hard drives, and external storage can be clumsy and slow I advise carving out a second partition on your primary hard drive. What better way to test drive 10.8 than with the native speed of an internal hard drive and none of the cables and extra baggage of an external enclosure.

The first thing you will need to do is backup your primary hard drive using cloning software like SuperDuper. That way you can restore everything back to the way it was in case anything should go awry. Just remember to test you backup by booting from it before repartitioning your boot drive. 1

Once your bootable backup is complete and its integrity tested, it is time to take stock of your primary hard drive and make sure you have enough room for an additional installation of Mac OS X. I recommend preserving at least 20GBs of free space to achieve this goal. A fresh install of 10.8 will take about 6GBs without a sleepimage, and you want to have extra room for installing applications, personal files, and swap space.

Once you have over 20GBs free on your primary drive user Disk Utility to carve out a new partition by resizing your existing boot volume. You can do this by highlighting the internal disk in Disk Utility’s source pane and selecting the Partition tab. Drag the lower right hand corner of the Partition Layout map up to compact your primary partition and shrink its available size by 20GBs. 20GBs of free space should be enough for testing 10.8, but use your own discretion based on the size of your primary drive and your own individual needs. Once you have provided space for your new boot volume use the plus button to create a new Mac OS Extended (Journaled) partition while giving it a unique name2. Don’t forget to hit Apply when you are done.

You can use the procedure I outlined in Installing Mountain Lion Clean to create a 10.8 boot disc and install the latest version of Mac OS X on your new partition. Once the installation is complete you can switch between the two different versions of Mac OS X by using the Startup Disk preferences pane, or by holding down the Option key at startup. The built-in firmware on every Mac makes switching between operating systems easy, no boot loader required.

This procedure can be used for installing multiple operating systems on any Mac. Unlike a virtual machine there is no loss in performance while dual booting. You will get a realistic feel for how 10.8, Linux, or Windows runs on your Macintosh’s hardware.

If you want to share the same Home folder between two different versions of Mac OS X just right-click on your account in the Accounts preference pane while booted under Mountain Lion. Select Advanced Options, and choose the Home directory on you primary boot partition. Different versions of applications can share the same preference files without issue, but keep a close eye on any application that has seen a considerable update in the new version of OS X.3

I am always dual booting my Mac between different operating systems. It gives me the chance to try something new, the way it was intended to be used without the fear of losing the stability of my primary OS. As Mountain Lion matures I will keep my secondary partition up to date with new versions of the developer preview, waiting for the day when 10.8 is ready to become my primary operating system.


  1. You can do this by holding down the Option key at startup while the bootable clone is still connected to your Mac. 

  2. As Dan Benjamin suggested I named my Mountain Lion partition “MoLo.” I hear in Finnish it is slang for wang

  3. Mail, Contacts, Calendar, and Messages all seem to work fine sharing preference files between Lion and Mountain Lion. 

Newton