Clean Install from OS X Mountain Lion - Here's how

This practical tip shows you how to perform a clean install of OS X Mountain Lion. Apple no longer delivers this version of its operating system on data media, but with a few steps you can easily create your own installation medium to rebuild your system. Basically, you can also use the following instructions to cleanly install OS X Mavericks.

Clean Install from OS X Mountain Lion: The Preparation

In any case, you should back up your data and programs before reinstalling. The easiest way to do this is with Time Machine. Read the best tips about this backup method here. Then prepare your new installation.

1. Open the App Store and download Mountain Lion. Do not select the installation directly, just download the installation image to your Mac. If you do not see an option in this regard, simply cancel the installation process after downloading.
2. Open the Finder and switch to the Programs folder. Right-click on the "OS X Mountain Lion Installation" program with the right mouse button (or control-click and left-click).
3. In the context menu, select the item "Show package contents". A new Finder window now opens. Navigate to the "Contents / SharedSupport" folder. There you will find the "InstallESD.dmg" file.
4. Open the hard disk utility. You now need a USB stick with at least six gigabytes. Plug this into a free USB port.
5. Click on this USB stick in the hard disk utility. Use the "Partition" tab to create a partition with the "Mac OS Extended (Journaled) format. Under Options, set the partition scheme to “GUID partition table”. Finally click on "Apply". Your USB stick will now be deleted.
6. In the Disk Utility, select the partition you just created. Then click on the "Restore" tab. Select your USB stick as the target medium. Use drag & drop to insert the "InstallESD.dmg" file under "Source" and then select "Restore". You now have an installation media that you can use to perform a clean install of OS X Mountain Lion.

Clean Install from OS X Mountain Lion: The Installation

Check again that you have backed up all your data and programs. In the next step, your system will be completely deleted and set up again.

1. Restart your system with the installation USB stick inserted. Hold down the alt key until a selection menu is displayed. In addition to your old installation, you will also find the USB stick here. Select this and use it to boot the installation routine.
2. After the boot process, select the language. Then you come to the main menu of the installation routine. Select the hard disk utility and click “Next”.
3. In the Disk Utility, select the hard drive on which you want to install OS X Mountain Lion. Use the "Partition" tab to create a partition in the "Mac OS Extended (Journaled)" format with the "GUID partition table" option and apply the changes. The hard drive is now completely deleted. You can also find useful tips on the hard disk utility here.
4. Exit the hard disk utility. You will now find yourself in the main menu of the installation. Click on the actual reinstallation of Mountain Lion. In the installation, select the hard drive you just deleted as the target and start the installation.
5. After the installation is complete, reboot your system and make the necessary entries. The best way to restore your data or programs is to use the migration assistant from OS X Mountain Lion and select your Time Machine Backup as the source.

You have now set up your system and played back your important data and programs. The advantage over a normal update is that you don't have to deal with legacy issues from a previous installation.

In another practical tip, we will show you how to fine-tune the volume under Mountain Lion.