The OS X Mountain Lion installer that you download from the Mac App Store can perform both an upgrade install (the default) and a clean install. A "clean" install means that you start fresh, by erasing all of the data on the target drive. You can perform a clean install on your startup drive, on another internal drive or volume, or on an external drive or volume. The process is a bit more difficult to perform on a startup drive because Apple doesn't provide bootable media for the OS X Mountain Lion installer; instead, you download the OS directly to your Mac from the Mac App Store. Since you run the installer from your Mac, you can't 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.
What You Need to Perform a Clean Install of OS X Mountain Lion
- The Mountain Lion Installer, which is available from the Mac App Store. Your Mac must be running OS X Snow Leopard (10.6) or later to access the Mac App Store.
- The installer is downloaded to the /Applications folder. Please note: When you download Mountain Lion, the installer will start automatically. Be sure to 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 8 GB in size. You can squeeze Mountain Lion onto a smaller drive, but there won't be much space left for user data and applications. I recommend a minimum volume size of at least 60 GB, which will provide sufficient room for the OS, user data, apps, and some free scratch space. That's my personal minimum. Consider not just how much space you need right now, but how much space you might need in the future. I installed Mountain Lion on a 320 GB volume, which is about right for the type of work I do.
- 650 MB 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 re-install the OS if necessary.
- A current backup. Performing a clean install of OS X Mountain Lion will erase all of the data on the target disk. For most of us, this will be the Mac's startup drive.
If you haven't already performed a backup, you can find instructions in the following guides:
What Is 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, and then meet us on page 2 of this guide.