Perform a Clean Install of OS X Mountain Lion on Startup Drive

Erase your Mac and perform a clean install for a fresh start

OS X Mountain Lion Installer app

Coyote Moon, Inc.

OS X Mountain Lion can perform both an upgrade install (the default) and a clean install. A clean install means you start fresh by erasing all the data on the target drive. You can perform a clean install on your startup drive, another internal drive or volume, or an external drive or volume. The process is more challenging to perform on a startup drive because Apple doesn't provide bootable media for the OS X Mountain Lion installer. Because you run the installer from your Mac, you can't erase the startup drive and run the installer at the same time.

Although Apple ended support for Mountain Lion in August of 2016, it is still available for purchase at the Apple Store. Your purchase generates a download code that you enter in the Mac App Store to download the software. 

Fortunately, there are alternative ways to perform a clean install on a Mac when the target for the installation is the startup drive.

What You Need to Perform a Clean Install of OS X Mountain Lion

How to Perform a Clean Install of OS X Mountain Lion on a Startup Drive
Screen shot courtesy of Coyote Moon, Inc.
  • Your Mac must be running OS X Snow Leopard (10.6) or later to access the Mac App Store.
  • The installer downloads to the /Applications folder. When you download Mountain Lion, the installer starts automatically. Quit the installer and read the rest of this guide before starting the installation process.
  • A supported Mac. The OS X Mountain Lion minimum requirements guide lists the Macs that can run Mountain Lion, as well as recommendations beyond the minimum.
  • A target drive or partition. The target volume must be at least 8GB in size. You can squeeze Mountain Lion onto a smaller drive, but there won't be much space left for user data and applications. A minimum volume size of at least 60GB provides sufficient room for the OS, user data, apps, and some free scratch space.
  • 650MB of free space for the Recovery HD partition, which is a hidden partition created by the Mountain Lion installer. It contains a bootable system with utilities for performing basic disk repair, as well as the ability to reinstall the OS if necessary.
  • A current backup. Performing a clean install of OS X Mountain Lion erases all the data on the target disk. For most people, this is Mac's startup drive.

If you haven't already performed a backup, you can find instructions in the following guides:

The Target Drive for the Clean Install of Mountain Lion

This guide covers performing a clean install of Mountain Lion on a startup drive.

If you intend to install OS X Mountain Lion on a second internal drive or volume, or an external USB, FireWire, or Thunderbolt drive, then you need the How to Perform a Clean Install of OS X Mountain Lion on a Non-Startup Drive guide.

Before you can perform a clean install of Mountain Lion on a startup drive, you must create a copy of the Mountain Lion installer on bootable media. The choices are a DVD, a USB flash drive, or a bootable external drive.

The Create Bootable Copies of the OS X Mountain Lion Installer guide has all the information you need. Use the guide to prepare your bootable media.

Starting the Clean Install on a Startup Drive

OS X Mountain Lion - Starting the Clean Install on a Startup Drive
The Mac OS X Utilities window. Screen shot courtesy of Coyote Moon, Inc.

There are two methods for performing a clean install of OS X Mountain Lion.

Start Your Mac From the Bootable Mountain Lion Installer

If you're going to install Mountain Lion on your Mac's startup drive, you must first restart your Mac from a bootable copy of the installer because the startup drive is erased before you perform the installation. Do this using Disk Utility, which is included with the installer.

  1. Insert the bootable media or connect it to your Mac and then restart the Mac while holding down the Option key. This causes the Mac to display its built-in startup manager, where you select the device you want to boot from. Use the arrow keys to select the bootable Mountain Lion installer you created earlier and then press the Enter key to start the boot process. The Mac OS X Utilities window displays as if you are booting from the Recovery HD partition. There isn't a Recovery HD partition available yet, because you haven't installed the OS. That's why you made your own bootable media.
  2. Select Disk Utility from the list of options and click Continue.
  3. Select your Mac's startup volume from the list of devices in Disk Utility. If you never changed its name, the startup volume is listed as Macintosh HD. Be sure to select the volume name and not the device name, which is usually the name of the physical drive — 500GB WDC WD5, for example.
  4. Click the Erase tab.
  5. Confirm that Mac OS X Extended (Journaled) is selected in the Format drop-down menu.
  6. Give the startup drive a name or use the default name.
  7. Click the Erase button.
  8. When you are asked if you are sure you want to erase the drive, click Erase.
  9. Select Quit Disk Utility from the Disk Utility menu to return to the Mac OS X Utilities window.
  10. Select Reinstall Mac OS X from the list and click Continue.
  11. When the Install OS X window opens, click Continue.
  12. Your computer's eligibility must be verified before you can download and restore OS X. This happens because the bootable media you created doesn't contain all the files necessary for the install. The installer checks for any missing or new files it needs, downloads the files from Apple's servers, and then starts the installation process. Click Continue.
  13. Read through the license and click the Agree button.
  14. Click the Agree button a second time to confirm you didn't accidentally click the Agree button.
  15. The installer displays a list of drives you can install Mountain Lion on. Select the target drive (the startup drive you erased earlier) and click the Install button.
  16. The installer checks the Mac App Store for updates and any other files it needs. Enter your Apple ID and click Sign In.
  17. The installer copies the necessary files to the target disk and restarts the Mac.

Finishing the Clean Install Process on a Startup Drive

OS X Mountain Lion - Finishing the Clean Install Process on a Startup Drive
You can choose to transfer user data, applications, and other information from another Mac, PC, or hard drive. Screen shot courtesy of Coyote Moon, Inc.

Finishing a clean install of OS X Mountain Lion on a startup drive is a relatively simple process. The on-screen prompts provided by the installer take you through most of it, but there are a few tricky spots.

Completing the Installation

When your Mac reboots, a progress bar displays the time remaining in the installation. The time varies depending on the Mac, but it should be relatively short — less than 30 minutes in most cases. When the progress bar hits zero, your Mac automatically restarts, and you are prompted for information.

  1. After restarting, your Mac starts the system setup process, including creating an administrator account and creating an iCloud account (if you want one).
  2. Select your country from the list on the Welcome screen and click Continue.
  3. Select your keyboard layout from the list and click Continue.
  4. Choose whether to transfer user data, applications, and other information from another Mac, PC, or hard drive. You can also choose not to transfer data now. Select the Not Now option. You can transfer data later using the Migration Assistant included with the OS. This way, you ensure that your Mac is up and running with Mountain Lion without any problems before you spend the long time it takes to transfer your data. Make your selection and click Continue.
  5. Enable location services. This feature allows your apps to figure out your approximate location and then use that data for various purposes, ranging from mapping to advertising. Safari, Reminders, Twitter, and Find My Mac are just a few of the apps that use location services. You can enable location services at any time, so you don't have to decide now. Make your selection, and click Continue.
  6. The installer asks for your Apple ID. You can skip this step if you want, but if you supply the information now, the installer preconfigures iTunes, the Mac App Store, and iCloud. It also pulls account information you provided in the past to make the registration process easier. Make your selection, and click Skip or Continue.
  7. Review the terms and conditions for various services included with OS X Mountain Lion. Click Agree.
  8. You know the drill; click Agree again.
  9. Allow the installer to set up iCloud on your Mac. You can do this yourself later, but if you plan to use iCloud, let the installer take care of the setup process. Make your selection and click Continue.
  10. If you chose to have the installer set up iCloud, your contacts, calendars, reminders, and bookmarks are uploaded to and stored in iCloud. Click Continue.
  11. Set up Find My Mac, a service that uses location services to determine where your Mac is if you misplaced it or it is stolen. With Find My Mac, you can remotely lock the Mac or erase its drive, which is handy for lost or stolen Macs. Make your selection, and click Continue.
  12. If you chose to set up Find My Mac, you are asked if it's OK for Find My Mac to display your location when you attempt to locate your Mac. Click Allow.
  13. Create your administrator account. Enter your full name. The account name defaults to your full name with all the spaces and special characters removed. The account name is also all lowercase letters. Accept the default account name or create your own account name if you prefer, using no spaces, no special characters, and all lowercase letters. You also need to enter a password. Don't leave the password fields blank.
  14. You can choose to allow your Apple ID to reset the administrator account password. If you occasionally forget important passwords, this may be a good option for you. You can also choose whether or not a password is required to log in to your Mac. Make your selections and click Continue.
  15. The Time Zone map appears. Select your location by clicking on the map or by selecting an area from the drop-down menu. Make your selections and click Continue.
  16. Registration is optional; click the Skip button, if you don't want to register. Otherwise, click the Continue button to send your registration information to Apple.
  17. A Thank You, screen displays. All you need to do now is click Start Using Your Mac.

The Desktop appears. It's almost time to start exploring your new operating system, but first, a little housekeeping.

Check for Updates for OS X Mountain Lion

You'll probably be tempted to start checking out Mountain Lion immediately, but before you do, it's a good idea to look for software updates.

Select Software Update from the Apple menu and follow the instructions for any updates listed. After you install any available updates, you're ready to go.