Dual Boot Mountain Lion And Lion On Your Mac

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Mountain Lion was released two days ago (25th July) with a tons of awesome features like iMessage, Airplay Mirroring, Notification Center and many more. The best part is it costs only $19.99 to upgrade making it almost irresistible. But even though Apple has made the upgrade process seamless, not all of us can replace our existing OS without actually testing it and making sure all our apps work perfectly. This is especially crucial for enterprise users. So, what better way to test it than to run Mountain Lion side by side to your existing OS. This can be achieved by dual booting your existing OS and Mountian Lion together and in this article we show you exactly that.

Download Mountain Lion

You can download Mountain Lion from the Mac App store after buying it (also see http://www.apple.com/osx/how-to-upgrade/ for details). The download takes little more than an hour over a broadband connection and is about 4.3 GB in size. The Downloaded file “Install OS X Mountain Lion.app” can be located under Finder > Applications (same as /Applications). Once the download is complete, the Mountain Lion setup wizard is launched (see below picture). At this point make sure to Control click or Right click the Mountain Lion setup icon on the Dock and select “Quit”. This will prevent ML from installing and deleting the downloaded file. Optionally, you can also copy the Mountain Lion installation file you just downloaded to a different location for later use.

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Preparing Your Mac For Dual Boot

Make sure to backup all your data. Although, your existing OS and data is not touched, backing-up is highly recommended, just in case something were to go wrong. Now, launch “Disk Utility” from Finder > Application > Utilities > Disk Utility. Then select your Hard Drive, usually the first Drive in the list, and select the “Partition” tab, as shown in the picture below.


Now, click the plus sign below the partition layout and split the selected partition into two. You can fine tune the size of the new partition by selecting it and entering the size you desire in the “Size” box. You could also drag the bottom right corner of your current partition to resize it and create a new partition from the remaining free space. You will need 15 GB partition at the minimum to install Mountain Lion, but if you plan on running both Mountain Lion and your existing OS for a long time, then I recommend creating at least a 50 GB partition. You can also name your partition in the “Name” box. Once done, click “Apply”. To make any correction, click “Revert” and start over.

Once you click “Apply” you will be asked to confirm your decision one last time, this is the time to abort if you are unsure. If you are sure click “Partition”. Finally, you should now see the new partition you created. In case you run into issues, see my article on “Partitioning A Hard Drive On Mac” for help and detailed explanation.

Installing Mountain Lion On New Partition

Now that you have downloaded ML and partitioned your Drive, you are ready to install the ML on the newly created partition which will ultimately allow you to dual boot. Next, relaunch the installer by opening the earlier downloaded file “Install OS X Mountain Lion.app” under Finder > Applications (same as /Applications) and agree to the license agreement. On the next screen you can click “Show All Disks” button and choose the disk on which you want to install ML. Make sure to pick the newly created partition from list of options and not the partition on which your current OS is installed, see below pictures for example.

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Optionally, you could also choose to install ML on an external Hard Drive by selecting it from the menu. Once you select the Drive and click “Install”, the wizard will prepare the Drive for install and ask you to restart the system for Mountain Lion installation. Save and close all other programs and click “Restart”.

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After the system reboots, Mac OS Base System gets loaded, now click the “Continue” button on “Install OS X” screen and agree to the License.

mountain lion install 2

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Now you will again be prompted to select the disk on which you want to install Mountain Lion. Make sure to pick the newly created partition from list of options and click “Install”, see below picture.

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Finally, Mountain Lion gets installed on the newly created partition and the system is rebooted. Note, the install takes about 15 to 20 minutes.

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Customizing Your Preferences

After the reboot, you will be guided through a personalization wizard and are asked few questions specific to you, like your  Location, Time Zone, Network details and the User Name you would like. Once you are done with this, the setup is complete and you should see a “Thank You” screen as shown below. Click “Start Using Your Mac” button to begin using Mountain Lion.

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Final Note, in case you want to boot into your old OS, make sure to keep the “Options” key pressed down. This will bring up the Startup Manager, at this point you can select your first partition (usually the first disk in the menu) to boot into your old OS.


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About Vijay

Computer junky for 20 plus years and still obsessed with it. I enjoy exploring and tweaking popular operating systems and tinkering with computer hardware. Professionally, I work as a Storage Systems Administrator. When not on my computer, I love to travel, shop, read and hang out with my family and friends.


  1. Fred Irkinfentl says:

    I have an old 1TB drive as Drive #2 in my Mac Pro. Can I just use that without partitioning my Snow Leopard (main) drive?

    • Hi Fred,
      Yes, you certainly can use your Drive # 2 to install Mountain Lion. Just select Drive # 2 when prompted to pick a drive during the install ( slides 5 and 9 from the top ). Note: Make sure to backup all data on Drive # 2, since if you select the drive as is, the entire drive space will be used to install ML.


      P.S: Make sure to subscribe or follow us on Facebook and Twitter, to stay posted about similar articles in the future.

  2. Hey Vijay,
    I’m in a similar but different situation.

    I just bought a Mac Pro 3,1 8 Core system. Unfortunately it came with Mountain Lion…

    I’ve since installed FCP Studio 2, and FCP 7 which has history of not running well on ML as he case has proven to be.
    Since ML is already installed, do you know how I install a dual boot system with Snow Leopard on another partition / hard drive etc?
    I get a kernel panic when trying to boot from the SL on an external drive, so am having no luck there…
    I took a Sata drive out of my external Lacie enclosure and tried it in Internal Bay 3 (&4) but it isn’t being recognised…(maybe it’s too old!?!) so I’m not sure of the issue there…

    I’d appreciate any advice you can give me.

    Thanks in advance.

    • Hi Rob,

      You will need the Snow Leopard install disc. You could partition you internal drive and install SL on the new partition. Alternatively, you could also install it on a second drive or even an external drive.

      Even though I personally haven’t tried doing it, the process should be quite similar. During boot-up, keep the options key pressed and select to boot from the optical drive (make sure SL install disc is inserted). Just select the appropriate drive during the install process.

      Another option is to install SL on a virtual machine created using VMware Fusion or Parallel.

      I highly recommend that you backup all your data before proceeding incase things do not go as expected.

      Let me know how it goes, good luck :)

      • Hi Vijay,
        Thanks for the advice. I got myself a copy of SL on disc. I have already formatted a partition for SL install so it’s ready.

        I’ve tried restarting holding option key (and also tried with C held) but whenever I select the Snow Leopard install disc I get a Kernel panic, and obvious freeze. So I’m stuck and can’t work out what I have to do to get SL installed.

        I do have a Caldigit Raid Card installed so am wondering if that’s possibly causing startup issues (I’ve sent them a message) or if it’s just ML being a pain.

        Any suggestions on what else I can try?