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

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

What to Know

  • Restart the Mac from the bootable installer while holding down the Option key. Select the installer and press Enter.
  • Select Disk Utility > Continue. Choose the Mac startup volume and select Erase. Confirm Mac OS X Extended (Journaled).
  • Choose Erase. Quit Disk Utility and select Reinstall Mac OS X.

This article explains how to perform a clean install of OS X Mountain Lion on your Mac's startup drive. It includes information on the decisions you make when performing a clean install.

Install From a 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, 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 is not a Recovery HD partition available yet, because you have not installed the OS. This is why you made your own bootable media.

  2. Select Disk Utility from the list of options, then select Continue.

    OS X Mountain Lion Disk Utility
  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, such as 500GB WDC WD5.

  4. Select the Erase tab and confirm that Mac OS X Extended (Journaled) is selected in the Format drop-down menu.

  5. Give the startup drive a name or use the default name, then select Erase.

  6. When you are asked if you are sure you want to erase the drive, select Erase, then select Quit Disk Utility from the Disk Utility menu to return to the Mac OS X Utilities window.

  7. Select Reinstall Mac OS X from the list, then select Continue.

  8. When the Install OS X window opens, select Continue.

  9. Your computer's eligibility must be verified before you can download and restore OS X. This happens because the bootable media you created does not 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. Select Continue.

  10. Read through the license agreement (or don't), then select Agree. Select Agree once more to confirm.

  11. The installer displays a list of drives on which you can install Mountain Lion. Select the target drive (the startup drive you erased earlier), then select Install.

  12. The installer checks the Mac App Store for updates and any other files it needs. Enter your Apple ID and select Sign In. The installer copies the necessary files to the target disk and restarts the Mac.

Finishing the Clean Install Process on a Startup Drive

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

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.

When your Mac reboots, a progress bar displays the time remaining in the installation. The time varies depending on the device, 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 don't already have one). Select your country from the list on the Welcome screen, then select Continue.

  2. Select your keyboard layout from the list, then select Continue.

  3. 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 Not Now. You can transfer data later using the Migration Assistant included with Mountain Lion. This way, you can ensure that your Mac is up and running without any problems before committing to the lengthy process of transferring data. Make your selection, then select Continue.

  4. Choose whether to enable location services. This feature allows your apps to figure out your approximate location and then use that data for various purposes, including mapping, advertising, and device location. 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, then select Continue.

  5. The installer asks for your Apple ID. You can skip this step if you'd like, but if you supply the information now, the installer configures 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 by selecting Skip or Continue.

  6. Review the terms and conditions for the various services included with OS X Mountain Lion, then select Agree. Select Agree once again to confirm.

  7. 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, then select Continue.

  8. If you chose to have the installer set up iCloud, then your contacts, calendars, reminders, and bookmarks will be uploaded and stored in iCloud. Select Continue.

  9. Set up Find My Mac, a service that uses location services to determine where your Mac is if it is lost of misplaced. 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, then select Continue.

  10. 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. Select Allow.

  11. 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.

  12. 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 into your Mac. Make your selections, then select Continue.

  13. Registration is optional. Select the Skip button, if you don't want to register now. Otherwise, select Continue to send your registration information to Apple.

  14. A Thank You screen displays. All you need to do now is select 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.

Although Apple ended support for Mountain Lion in August of 2016, it is still available for purchase from the Apple Store.

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

OS X Mountain Lion
(10.8) can perform both an upgrade install 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 does not provide bootable media for the OS X Mountain Lion installer. Because you run the installer from your Mac, you cannot erase the startup drive and run the installer at the same time.

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

How to Perform a Clean Install of OS X Mountain Lion on a Startup Drive
Screen shot courtesy of Coyote Moon, Inc.
  • 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. You also need a Mac running OS X Snow Leopard (10.6) or later to access the Mac App Store.
  • 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 space.
  • 650MB of free space for the Recovery HD partition. This 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.

The installer downloads to the /Applications/ folder. When you download Mountain Lion, the installer starts automatically.

Don't Forget the Backup

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 should consult the following 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. Consult the following guide to prepare your bootable media:

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