How to install Mountain Lion over Leopard

[Editor’s note: This article is part of our series of articles on installing and upgrading to Mountain Lion.]

As I explained in my guide to installing Mountain Lion, one of the requirements for installing OS X 10.8 is that you already have at least Snow Leopard (Mac OS X 10.6) installed. (Specifically, Mountain Lion requires OS X 10.6.8 or later.) The main practical reason for this requirement is that Mountain Lion is available only via the Mac App Store, and the Mac App Store debuted in Mac OS X 10.6—in other words, you need Snow Leopard or Lion just to be able to purchase and download Mountain Lion.

But once you’ve got your copy of Mountain Lion, can you install it onto a Mac or an external drive containing Leopard (Mac OS X 10.5)?

The license agreement

The software license you agree to when you install Mountain Lion states that you can “download, install, use and run for personal, non-commercial use, one (1) copy of the [the OS] directly on each Apple-branded computer running OS X Lion or OS X Snow Leopard…that you own or control.” In other words, if your Mac shipped with Lion or Snow Leopard, you can install Mountain Lion. If your Mac shipped with Leopard or Tiger (Mac OS X 10.4), but you later purchased and installed Snow Leopard or Lion, you can install Mountain Lion. If your Mac doesn’t at least have Snow Leopard installed, you can’t install Mountain Lion.

That seems pretty clear. But what if, for example, you’ve got a family-pack license for Snow Leopard, and you’ve got a Mac that shipped with Leopard but that’s never been upgraded to Snow Leopard or Lion? The Mountain Lion license agreements say that even if that Mac is compatible, you can’t upgrade to 10.8 until you first install at least Snow Leopard.

This is just one scenario—I can think of a number of situations in which you might have Leopard on a Mac or an external drive, along with a valid license for Snow Leopard, and you’d rather not take the interim step of installing Snow Leopard just to upgrade to Mountain Lion. Having performed this two-step upgrade many times while researching our various Mountain Lion-installation articles (and last year while writing our Lion-installation articles), I can tell you that it’s a real hassle.

The practical question

But lets take a step back. While the letter of the law says you need to install at least Snow Leopard before installing Mountain Lion, the spirit of the law seems to be that a particular Leopard-equipped Mac just needs a license for Snow Leopard or Lion before you can upgrade it. In other words, in our view, you should be well within your rights to install Mountain Lion on any of your computers for which you have a valid, current Snow Leopard or Lion license—even if you don’t actually install Snow Leopard first. So then the question becomes whether there are any technical reasons you can’t install Mountain Lion over Leopard.

In my testing with many Macs, the Mountain Lion installer, like the Lion installer before it, refuses to install onto a drive containing Leopard; in fact, it refuses to install on any drive running a version of Mac OS X below 10.6.8, just as its official system requirements claim. The Mountain Lion installer will, however, install onto a blank drive, so Mountain Lion clearly doesn’t need any of Snow Leopard’s files or settings.

You may be thinking, “If it will install onto a blank drive, I’ll just copy the installer to my Leopard-equipped Mac, connect an empty drive, install the new OS there, and then use Setup/Migration Assistant to move my files over.” Alas, while the Mountain Lion installer will freely install onto a blank drive, the Mountain Lion installer itself must be run from within Snow Leopard, Lion, or Mountain Lion.

So how can you install Mountain Lion over Leopard? There are three ways: the official way, the brute-force method, and the quick-but-techie way. Whichever method you choose, you should—as with any OS installation—be sure to have an up-to-date, tested backup of your drive before you begin.

Note that there are actually two Mountain Lion-compatible Macs—the Mid 2007 iMac and the Mid/Late 2007 MacBook Pro—that shipped with Tiger [Mac OS X 10.4]. If you’ve got one of these Macs, still running Tiger, and you’re determined to upgrade it to Mountain Lion, the first two methods below (“The official way” and “The brute-force method”) will work; the third method (“The quick-but-techie way”) will not.

The official way

As I explained above, Apple’s official policy is that if you want to install Mountain Lion over Leopard—assuming, of course, the Mac in question meets the system requirements—you must first install Snow Leopard, purchasing it for $29 if necessary, and then install Mountain Lion. This approach works fine, it’s fairly easy to do (if a bit time-consuming), and it gets the Apple seal of approval.

The brute-force method

What if you don’t want to install Snow Leopard first, or if you don’t have your Snow Leopard disc handy? I’m not being coy here—perhaps you’ve misplaced the disc, or maybe you’re on the road and you’ve got your Mac’s original (Leopard) disc with you as an emergency boot disc, but you don’t have your Snow Leopard upgrade disc.

As I mentioned above, the Mountain Lion installer will let you install onto a bare drive as long as the installer itself is run under Snow Leopard, Lion, or Mountain Lion. This means that as long as you have a good backup; a 5GB-or-larger thumb drive or external drive; and either an already-downloaded copy of the installer or access to a Mac running Snow Leopard, Lion, or Mountain Lion, you can perform a bit of installer razzle-dazzle.

Specifically, the procedure involves erasing your Mac’s drive, installing Mountain Lion onto it, and then importing all your data from your backup. (If this sounds a lot like a clean install, that’s because it’s essentially the same process.) Here’s are the steps to take:

1. Make sure you have an up-to-date backup—either a Time Machine backup or a clone backup using a utility such as SuperDuper or Carbon Copy Cloner—of your Leopard Mac’s drive. (For this purpose, I recommend a clone.) Be sure to test this backup to verify that it has your latest data: In the case of a Time Machine backup, try restoring some important data from the backup; in the case of a clone backup, boot from the clone to make sure it boots and that it contains all your data.

2. Use the computer running Snow Leopard or later to download the Mountain Lion installer from the Mac App Store. (If you’ve already got your copy of the Mountain Lion installer, skip this step.)

3. Create a bootable install drive using the instructions for creating a bootable Mountain Lion install drive.

4. Boot your Leopard Mac from that new install drive. When you do so, you’ll find yourself with a screen called OS X Utilities with several options. This is the same screen you’ll see if you boot your Mac in recovery mode.

5. Select Disk Utility and click Continue, then use Disk Utility to erase your Leopard Mac’s internal drive. To do so, select that drive on the left, click Erase on the right, choose Mac OS X Extended (Journaled) from the Format pop-up menu, and click Erase. Warning: This step erases all the data on your Mac’s drive, which is why you needed that backup!

6. When the erase procedure is finished, quit Disk Utility to get back to the OS X Utilities screen.

7. Select Reinstall OS X and click Continue to launch the OS X installer and install Mountain Lion on your Mac’s internal drive.

8. After your Mac restarts, installation finishes, and you proceed through the setup process, watch for the Transfer Information To This Mac screen. You’ll use the third option, From Time Machine Or Another Disk, to transfer all your files from your backup to your new installation of Mountain Lion.

When the transfer process is finished, you’ll be able to log in to Mountain Lion with all your accounts and data intact.

The quick-but-techie way

If you’re comfortable diving into the OS and editing a .plist file, this is the fastest way to install Mountain Lion over Leopard, although, as with the previous method, you’ll need to be able to boot from a Snow Leopard, Lion, or Mountain Lion drive to run the installer.

As I mentioned above, the Mountain Lion installer refuses to install over Leopard Mac. But how does the installer know your drive contains Leopard and not Snow Leopard or later? It turns out that the installer simply checks a particular file—/System/Library/CoreServices/SystemVersion.plist—on the destination disk to check the version of OS X currently installed on that disk.

Which means that if your Mac is running Leopard, and you’re feeling adventurous, you can edit the SystemVersion.plist file so that it claims you’re running, say, 10.6.8. The Mountain Lion installer—which will still need to be run on a Mac running Snow Leopard or later—will then install over Leopard without the slightest complaint. Here’s how to do that:

1. On your Leopard-equipped Mac, navigate to /System/Library/CoreServices.

2. Using a text editor that lets you enter an admin-user name and password to edit system-level files—such as the non-Mac App Store version (13MB download link) of TextWrangler—open the file called SystemVersion.plist.

3. Locate the ProductVersion key (not the ProductUserVisibleVersion key). Just below that is a string of numbers indicating the OS version; for example, on a Mac running OS X 10.5.8, it will read 10.5.8.

4. Change that number to 10.6.8, save the file (providing your admin-level username and password when prompted), and then shut down your Mac.

5. Boot your Mac from a drive running Snow Leopard or later that also contains the OS X installer. If you’ve created a bootable Mountain Lion install drive, just boot your Mac from it, and when the OS X Utilities screen appears, use the Reinstall OS X option to install onto your Leopard drive. Another approach would be to boot your Leopard Mac from an external drive containing Snow Leopard or later, and then run the installer from there. Yet another option, if you’ve got two Macs with FireWire or Thunderbolt, is to boot the Leopard Mac into Target Disk Mode and connect it to your Snow Leopard or later Mac, and then run the installer.

Whichever approach you take, when you’re done, you’ll have Mountain Lion on your previously Leopard Mac.

[Dan Frakes is a Macworld senior editor. His Leopard Macs have put up a good fight, but they’ve all given in to Mountain Lion.]

Updated 7/30/12, 8:30am, to include link to OS X 10.5-compatible version of TextWrangler.