[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.
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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.