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:
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.