How to Clean Install Mac OS X Mountain Lion

In the Windows’ analogy, this is like reformatting your Macbook or iMac and reinstalling a brand new version of OS X Mountain Lion. Unfortunately, in Mac, the process can be a little more complicated and less well known.

In this post, I’m going to show you how to erase all existing data, file sand applications, and reinstall a brand new copy of OS X Mountain Lion, bring your Mac operating syetem back to a clean slate.

Here is a breakdown of the steps involved:

  1. Download a copy of OS X Mountain Lion
  2. Make a bootable drive of OS X Mountain Lion with a USB drive
  3. Reboot, Clean up and Reinstall

Sounds good? Let’s do it.

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1. Re-download Mountain Lion Installer

First, grab hold of a copy of the Mountain Lion installer file. The installer you downloaded from the App Store automatically deletes itself after you’ve successfully upgraded (or install) your Mac.

Here’s how you can download and obtain a soft copy of Mountain Lion.

  1. Launch the App Store and go to the Purchases tab.
  2. purchase tab
  3. Look for OS X Mountain Lion, hold down the Option key and click Download.
  4. Re-enter your App Store credentials if required. If you’ve previously purchased a copy of Mountain Lion, don’t worry, you will not be recharged.
  5. download mountain iion

2. Create bootable OS X Mountain Lion

After your download is done, let’s proceed to making a bootable USB drive with your OS X Mountain Lion in it.

  1. go to Finder > Applications.
  2. Look for the file called "Install OS X Mountain Lion", Right-click and select Show Package Contents.
  3. show package content
  4. Navigate to Contents > SharedSupport, then look for the filename "InstallESD.dmg"
  5. mountain lion installer file
  6. Double-click InstallESD.dmg to open (or mount) it on your Desktop. (This may take a minute or two.)
  7. mount installer
  8. Plug your USB drive into your Mac and launch the Disk Utility application
  9. disk utility via spotlight
  10. Select the USB drive on the left menu, Erase tab. Under Erase tab, select Mac OS Extended (Journaled) format and click the Erase button.
  11. format usb as mac extended
  12. Once the USB drive is formatted, re-select the formatted USB drive from the left menu, and click the Restore tab.
  13. select restore tab
  14. Now, we are going to create a bootable OS X Mountain Lion in the USB drive. Drag the formatted drive from the left menu to the Destination field, then drag "Mac OS X Install ESD" on the left menu to Source and click the Restore button.
    restore source destination

The installation may take about 10 – 20 minutes and once it is done you’ll have yourself a USB drive with a bootable Mountain Lion.

3. Reboot, Clean up and reinstall

So we’ve downloaded a new copy of Mountain Lion, copied it over to the USB drive and made it bootable.

Now, we will attempt to remove everything from the current OS and reinstall a new copy of OS X Mountain Lion.

  1. Restart your Mac, hold down the Option key when you see the Apple logo. When you see the screen in the screenshot below, select Mac OS X on the right.
  2. restore source destination
  3. You will then be taken to the OS X Utilities dialogue box. Select Disk Utility.
  4. restore source destination
  5. At the Disk Utility screen, select your Mac’s hard drive, then click Erase tab and select Mac OS Extended (Journaled) as format.
  6. restore source destination
  7. Click the Erase button button and everything (including files, applications, settings, etc) in your Mac will be completely deleted.
  8. When you are done, you will be directed back to the OS X Utilities dialogue. Select Reinstall OS X, the second option from the list.
  9. restore source destination

That’s it!

Your Mac will now reinstall from the USB drive and when it is done you will be presented with new copy of OS X Mountain Lion.