The OS X Mountain Lion installer offers two installation options: an upgrade install (the default) and a clean install. A "clean" install erases all of the data on the target drive, so you start with a clean slate.

You can perform a clean install on a startup drive, another internal drive or volume, or an external drive or volume. In this guide, we're going to perform a clean install of Mountain Lion on a non-startup drive, which includes all of the aforementioned options except a startup drive.

If you want to install Mountain Lion on a startup drive, follow the instructions in our How to Perform a Clean Install of OS X Mountain Lion on a Startup Drive guide.

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

• The Mountain Lion Installer. You can download the installer from the Mac App Store. You must be running OS X Snow Leopard (10.6) or later to access the Mac App Store.
• The installer will be downloaded to your /Applications folder. When the download finishes, the installer will launch automatically. Quit the installer and read the rest of this guide before you begin the installation process.
• A supported Mac. Our OS X Mountain Lion Minimum Requirements guide lists the Mac models that can run Mountain Lion; it also makes recommendations beyond the minimum requirements.
• A target drive or partition. The target volume must be at least 8 GB in size. While you could probably squeeze Mountain Lion onto a smaller drive, you wouldn't have much room left for user data and applications; where's the fun in that? I recommend at least 60 GB, which will accommodate the OS, user data, and apps, as well as provide some scratch space for apps that need a little room to work in. When deciding just how much drive space to allot for the installation, think about what you might need in the future, not just what you need right now. I installed Mountain Lion on a 320 GB volume, which is perfect for me, but may be too much (or too little) for you.

• 650 MB of free space for the Recovery HD partition. This is a hidden partition created by the Mountain Lion installer. It holds a bootable system with utilities that can perform basic disk repair, as well as re-install the OS, if necessary.
• A current backup. If there's any data on the target drive that you want to save, be sure to back it up first. Note: While it's not necessary to erase the drive, as long as there isn't a system on it, by definition, a clean install starts with a drive or volume that contains no data.

If you haven't already backed up your data, or it's been a while since you've performed a backup and you're not sure you remember how to do it, you can find instructions in the following guides:

Mac Backup Software, Hardware, and Guides for Your Mac

Time Machine - Backing Up Your Data Has Never Been So Easy

Back Up Your Startup Disk Using Disk Utility

What Is the Target Drive for the Clean Install of Mountain Lion?

This guide covers performing a clean install of Mountain Lion on a secondary internal drive or an external USB, FireWire, or Thunderbolt drive.

If you want to perform a clean install of Mountain Lion on your startup drive, you'll find complete instructions in our How to Perform a Clean Install of OS X Mountain Lion on a Startup Drive guide.

Because you're not installing Mountain Lion on a startup drive, there's no current system data (or any other data) on the drive. The installer will set up all of the necessary files for the OS. It will also create the administrator account, create an iCloud account (optional), and set up the Find My Mac service (also optional).

Launch the OS X Mountain Lion Installer

Create Your Administrator Account

Registration

1. Before you begin, quit all apps.

1. Launch the Install OS X Mountain Lion app, located in the /Applications folder.
2. When the Install OS X window opens, click the Continue button.
3. Read through the license and click the Agree button.
4. Click the Agree button again, to show you really mean it.
5. By default, the installer will select your current startup drive as the target for the installation. Click the Show All Disks button.
6. A list of available disks will display. Select the target disk for the installation, and click Install.
7. You will be asked for your administrator account password. Enter the information, and click OK.
8. The installer will copy the necessary files to the target disk, and then restart your Mac.
9. When your Mac finishes rebooting, a progress bar will display the amount of time remaining in the installation. The time will vary, depending on the Mac, but it should be relatively short; less than 30 minutes in most cases. When the progress bar reaches zero, your Mac will restart.
10. The installer will then begin the process of setting up the system, including creating an administrator account, creating an iCloud account (if you want one), and setting up the Find My Mac service (if you want to use it).

1. When the Welcome screen displays, select your country from the list, and click Continue.
2. Select your keyboard layout from the list, and click Continue.
3. You can transfer user data, applications, and other information from another Mac, PC, or hard drive now, or you can transfer them later, using the Migration Assistant included with the OS. I recommend selecting the Not Now option, and taking a little time to ensure that the installation went smoothly, and that your Mac isn't having any obvious problems with Mountain Lion. Transferring data with the Migration Assistant can be a time-consuming process; it's better to find out if there are any problems first than to go through the data transfer process twice. (Of course, there are never any guarantees.) Make your selection, and click Continue.
4. You can enable the location services feature, if you wish. This feature lets your apps determine your approximate location and then use that information for various purposes, from useful (mapping) to potentially annoying (advertising). Safari, Reminders, Twitter, Time Zone, and Find My Mac are just a few of the Apple and third-party apps that can use location services. You can enable (or disable) location services at any time, so you don't have to decide now. Make your selection, and click Continue.
5. The installer will ask for your Apple ID. You can skip this step, if you wish, but if you provide the information, the installer will pre-configure iTunes, the Mac App Store, and iCloud for you. It will also gather account information you've provided in the past, which will make the registration process easier. Make your selection, and click Skip or Continue.
6. The terms and conditions for the various services included with OS X Mountain Lion will display. These include the OS X license agreement, iCloud terms, Game Center terms, and Apple's privacy policy. Read through the information, and click Agree.
7. You know the drill; click Agree again.
8. You can let the installer set up iCloud on your Mac, or you can do it yourself later. If you plan to use iCloud, I recommend letting the installer take care of the setup process for you. Make your selection, and click Continue.
9. If you chose to have the installer set up iCloud, it will upload your contacts, calendars, reminders, and bookmarks to iCloud. Click Continue.
10. You can set up Find My Mac now, leave it for later, or not use it at all. This feature uses location services to find your Mac if it's gone missing. If you've misplaced your Mac, or you think it may have been stolen, you can also use Find My Mac to remotely lock your Mac or erase its hard drive. Make your selection, and click Continue.
11. If you chose to set up Find My Mac, you'll be asked if it's okay for Find My Mac to display your location when you attempt to locate your Mac. Click Allow.
12. The next step is to create your administrator account. Enter your full name. The OS will automatically format it as fullname; all lowercase letters, with all spaces and special characters, such as apostrophes, removed. I recommend accepting the default account name, but you can create your own account name, if you prefer. It must follow the default format, though: no spaces, no special characters, and all lowercase letters. You also need to enter a password; don't leave the password fields blank.
13. You can choose to allow your Apple ID to reset the administrator account password. I don't generally recommend this, but if you're not the best at remembering passwords, this may be a helpful option for you.
14. You can also choose whether or not a password is required to log in to your Mac. I highly recommend this option if you're using a portable Mac.
15. Make your selections, and click Continue.
16. The Time Zone map will appear. Click on the map to select your location. To refine your location, click the drop-down chevron at the end of the Closest City field. Make your selections, and click Continue.
17. Registration is optional. You can click the Skip button, or click the Continue button to send your registration information to Apple.
18. A Thank You screen will display. Click the Start Using Your Mac button. When the Desktop appears, you can start using your new OS, but I recommend doing one more thing first.

Update OS X Mountain Lion

You'll be tempted to start exploring your new OS right away, and I don't blame you. But it's a good idea to check for and install any available software updates; then you can enjoy your new OS without interruption.

Select "Software Update" from the Apple menu, and follow the instructions for any updates listed. Restart your Mac, and you're in business.