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.
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. 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. 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.
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.
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.
Don’t Forget the Backup
If you haven’t already performed a backup, you can find instructions in the following guides:
Mac Backup Software, Hardware, and Guides for Your Mac Time Machine, Backup Software You Should Be Using Back Up Your Startup Disk Using Disk Utility
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:
How to Perform a Clean Install of OS X Mountain Lion on a Non-Startup Drive
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:
Create Bootable Copies of the OS X Mountain Lion Installer
