Upgrade Install of OS X Mountain Lion

Move up to Mountain Lion without losing your personal data

Mountain lion

Geoffrey Kuchera / Shutterstock

There are several ways to install OS X Mountain Lion. This guide will show you how to perform an upgrade install, which is the default installation and the one that Apple thinks most Mac users will choose. It's not the only option, though. You can also perform a clean install, or install the OS from various types of media, such as a USB flash drive, a DVD, or an external hard drive. We'll cover those options in other guides.

of 03

Upgrade Install of OS X Mountain Lion

OS X Mountain Lion Installer app

Coyote Moon, Inc.

OS X Mountain Lion is the second version of OS X that can only be purchased through the Mac App Store. If you haven't yet upgraded to OS X Lion, the new distribution and installation methods may seem a bit foreign. On the plus side, Apple worked out most of the glitches on Lion, so you get the benefit of installing Mountain Lion using a well-understood and reliable method.

If you did upgrade to OS X Lion, you'll find most of the installation process to be very similar. Either way, this step-by-step guide will help ensure that you understand how everything works.

What Is an Upgrade Install of OS X Mountain Lion?

The upgrade install process lets you install Mountain Lion over your existing version of OS X, and still retain all of your user data, most of your system preferences, and most of your applications. You may lose some of your apps if they can't run under Mountain Lion. The installer may also change some of your preference files because certain settings are no longer supported or are incompatible with some feature of the new OS.

Before You Perform an Upgrade Install

Most of you won't have any problems with installing and using Mountain Lion, but there's a small chance that your particular combination of apps, data, and preferences will be the one that was never thoroughly tested before Mountain Lion was released. That's one reason why I highly recommend backing up your current system before you start the upgrade process. I prefer to have a current Time Machine backup, as well as a current clone of my startup drive. That way I can revert my Mac to the way it was configured before I started the installation, should I need to, and it won't take long to do it. You may prefer a different backup method, and that's fine; the important thing is having a current backup.

What You Need to Perform an Upgrade Install of OS X Mountain Lion

  • A copy of the Mountain Lion installer, which is available from the Mac App Store. You must be running Snow Leopard or later to access the Mac App Store, but you don't have to install Lion before you install Mountain Lion. Mountain Lion will install correctly as long as you're running OS X Snow Leopard or later on your Mac.
  • A destination volume for the installation. The Mountain Lion installer can work with internal drives, SSDs (Solid State Drives), or external drives with USB, FireWire, or Thunderbolt interfaces. Basically, any bootable device will work, but because this is a guide to an upgrade install, the target volume must be running OS X Lion or earlier. If your Mac doesn't meet this requirement, then the Clean Install guide is a better choice for you.
  • A minimum of 8 GB of free space; more space is better, of course.
  • A minimum of 650 MB of free space for the Recovery HD volume. This is a hidden volume that is created during the installation. The Recovery HD volume contains utilities to repair drives and to re-install the OS if you have problems with a drive.

If you have everything lined up, and you've ensured that you have current backups in place, let's begin the actual upgrade process.

of 03

Install OS X Mountain Lion — The Upgrade Method

OS X Mountain Lion Installer with destination disk selection.
The Mountain Lion installer selects your current startup drive as the target for the installation (the Show All Disks button is only visible if there are multiple drives connected to your Mac.).

Coyote Moon, Inc.

This guide will take you through an upgrade install of OS X Mountain Lion. The upgrade will replace the version of OS X you're currently running but will leave your user data and most of your preferences and apps in place. Before you begin the upgrade, make sure you have a current backup of all your data. While the upgrade process shouldn't cause any problems, it's always best to be prepared for the worst.

Installing OS X Mountain Lion

  1. When you purchase Mountain Lion from the Mac App Store, it will be downloaded to your Mac and stored in the Applications folder; the file is called Install OS X Mountain Lion. The download process also creates a Mountain Lion installer icon in the Dock for easy access and auto-starts the Mountain Lion installer. You can quit the installer if you're not ready to begin the installation process; otherwise, you can continue from here.
  2. Close any applications that are currently running on your Mac, including your browser and this guide. You can print the guide first by clicking the printer icon in the top right corner of the guide.
  3. If you quit the installer, you can restart it by either clicking its Dock icon or double-clicking the Install OS X Mountain Lion file in the /Applications folder.
  4. The Mountain Lion installer window will open. Click Continue.
  5. The license will display. You can read the terms of use or just click Agree to get on with it.
  6. A dialog box will ask if you really read the terms of the agreement. Click Agree.
  7. By default, the Mountain Lion installer selects your current startup drive as the target for the installation. If you want to install Mountain Lion on a different drive, click the Show All Disks button, select the target drive, and click Install. (The Show All Disks button is only visible if there are multiple drives connected to your Mac.)
  8. Enter your administrator password and click OK.
  9. The Mountain Lion installer will start the installation process by copying the necessary files to the selected destination drive, usually the startup drive. The amount of time this will take depends on how fast your Mac and the drives are. When the process is complete, your Mac will restart automatically.
  10. After your Mac restarts, the installation process will continue. A progress bar will display, to give you an idea of how much more time the installation will take. My installation took about 20 minutes; your mileage may vary.
  11. When the installation is complete, your Mac will restart again.

If you use multiple monitors, be sure to have all of the monitors turned on. During the installation, the progress window may display on the secondary monitor instead of your main monitor. You won't see the progress window if the display is turned off, and you may think something is going wrong with the installation. More importantly, if you can't see the progress window, you won't have any idea how long you have to wait before you can use your new OS.

of 03

Upgrade Install OS X Mountain Lion — Install Complete

Upgrade Install OS X Mountain Lion


Your Mac will restart automatically once the installation is complete. This is where many people get worried because the first startup with OS X Mountain Lion can take a long time. Mountain Lion analyzes your Mac's hardware, fills data caches, and performs other one-time housekeeping chores. This startup delay is a one-time event. The next time you start your Mac, it will respond as expected.

  1. When Mountain Lion is done, either the log-in screen or the Desktop will display, depending on whether you previously had your Mac configured to require log-in.
  2. If you didn't have an Apple ID set up for your current OS, the first time your Mac starts up with Mountain Lion you will be asked to supply an Apple ID and password. You can enter this information and click Continue, or skip this step by clicking the Skip button.
  3. The Mountain Lion license will display. This includes the OS X license, the iCloud license, and the Game Center license. Read the information or not, as you choose, and then click the Agree button.
  4. Apple will ask you to double-dog confirm the agreement. Click Agree again.
  5. If you don't already have iCloud set up on your Mac, you'll be given the option to use the service. If you wish to use iCloud, place a checkmark in the Set Up iCloud on This Mac box and click Continue. If you don't want to use iCloud, or you'd rather set it up later, remove the checkmark and click Continue.
  6. If you choose to set up iCloud now, you'll be asked if you want to use Find My Mac, a service that can locate your Mac on a map if you ever misplace it, or if it's stolen. Make your selection by placing or deleting a checkmark, and then click Continue.
  7. The installer will finish and present a Thank You display. Click the Start Using Your Mac button.

Update the Mountain Lion Software

Before you get busy checking out your new installation of OS X Mountain Lion, you should run the Software Update service. This will check for updates of the OS and many supported products, such as printers, that are connected to your Mac and may need updated software to work correctly with Mountain Lion.

You can find Software Update under the Apple menu.