It used to be that when upgrading to a major new version of OS X, installing over an existing OS X installationfor example, installing 10.3 over 10.2entailed some degree of risk, as existing applications, add-ons, and support files could conflict with the new OS. For this reason, many people used to perform a clean install: wiping your hard drive (after backing it up, of course), installing the latest version of OS X, and then either using Setup/Migration Assistant to restore your applications and data, or manually copying over your data and reinstalling programs. (The Mac OS X 10.2 installer actually included an Archive And Install option, which preserved your original OS in a special folder while installing a completely new, fresh copy of 10.3. This feature was eliminated in the Snow Leopard OS X 10.6 installer.)
But a new download-and-install procedure debuted with Lion (OS X 10.7) and continues with Mountain Lion (OS X 10.8)instead of a bootable installation DVD, you download the latest OS X installer to your Mac and install it from the same drive. As with Lion last year, many Mac users are asking two related questions: (1) Can you perform a clean install of Mountain Lion? and (2) Should you? Heres my take on each of these questions.
Can you perform a clean install of Mountain Lion?
First, the technical question: Given that the OS X 10.8 installer doesnt include an official clean-install option, is it possible to perform such an installation? The simple answer is: Yes. As explained in my main article on installing Mountain Lion, the installer will let you install the new OS onto a blank drive. So if you first back up your existing Snow Leopard or Lion installation and all your filesI recommend creating a bootable clone using SuperDuper or Carbon Copy Cloneryou can then boot from a bootable installer drive, erase your Macs normal startup drive, and install Mountain Lion on it. In fact, you can use the instructions in my article on how to install Mountain Lion over Leopard. Specifically, scroll down to the section called The brute-force method and perform Steps 1 through 7, substituting Snow Leopard or Lion for Leopardthe result is a clean install.
Once youve done this, if you want to use Setup/Migration Assistant to restore data from your backup, proceed with Step 8. If you truly want a clean start, youll instead need to manually copy your personal data from your backup to your new Mountain Lion installation, and then reinstall all of your software. (This is one situation in which the more apps youve purchased through the Mac App Store, the betteryou just launch the Mac App Store app and click a few buttons to automatically reinstall everything youve purchased.)
Should you perform a clean install of Mountain Lion?
OK, so you can, but should you? Prior to Snow Leopard, I generally recommended a clean install. But the Snow Leopard installer and Setup/Migration Assistant were pretty good about not transferring over incompatible software, and Lion and Mountain Lion have been even betterin fact, Lion and Mountain Lion even automatically detect some incompatible programs and system add-ons the first time you log in, as explained in my main installation article.
What about stuff the installer and Setup/Migration Assistant dont catch? In my experience installing 10.8 many times over a variety of existing Lion, Snow Leopard, and even Leopard installations, Ive had little trouble that I could trace directly to incompatibilities with transferred code, and upgrading to Mountain Lion has gone even more smoothly than the many Lion upgrades I performed last year. Based on that experience, and similar reports from my Macworld colleagues, I feel comfortably saying that as long as youve properly prepared your Mac before installing Mountain Lion, you should be just fine installing directly over Lion or Snow Leopard. (Because Mountain Lion and Lion have so much code in common, upgrading from Lion to Mountain Lion seems to entail even less risk than upgrading to from Snow Leopard.)
There are, however, a couple situations in which you might consider a clean install. The first is if youve done some funky partitioning of your Macs startup drive that prevents the Mountain Lion installer from creating the special Recovery HD partition. Given how useful recovery mode is in Mountain Lion, I recommend performing a clean install (with a good backup!) just so you can erase your Macs drive and restore it to a standard configuration that will allow the installer to create the Recovery HD partition. (If you dont want to manually re-install everything afterwards, you can use Setup or Migration Assistant to transfer your data, applications, and the like from your backup to the new installation, as described above.)
The other is if youve been using your Mac for a while, installing and deleting lots of apps and OS add-ons, and your hard drive has become littered with lots of unnecessary gunk and cruft: orphaned application-support and preference files, abandoned preference panes, and the like. A new major version of OS X is a great opportunity to do some spring cleaning, so to speak. Of course, if you perform a clean install for this purpose, you dont want to use Setup or Migration Assistant to bring over everything from your backup. Instead, you should manually copy your personal data and then reinstall just those apps and add-ons you actually use. (Macworld contributor Joe Kissell talks extensively about such procedures in Take Control of Upgrading to Mountain Lion.)
[Dan Frakes is a Macworld senior editor. Hes very, very happy the days of frequent clean installs are behind us.]