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 Mac OS X Mountain Lion. Unfortunately in Mac the process can be a little more complicated and less well known. But first, why should you clean install the Mac OS X Mountain Lion at all?

Here are couple of reasons we can think of: One, your existing Mac OS X inherited documents, files and applications that take up a lot of unnecessary storage space. To optimize your Mac, it is quite necessary to reinstall and start from a clean slate.

mountain lion

Two, you are thinking about trading or gifting your machine to someone else, but you are concerned with the remnants of files, profiles and various credentials that if mishandled, may lead to a security leak. A clean install will give you peace of mind.

With that cleared up, in this article we will show you how to erase existing data and reinstall a brand new copy of Mountain Lion. Here is a breakdown of the steps involved:

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

You ready? Let’s do it.

1. Re-download Mountain Lion

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

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

Launch App Store, and go to the Purchases tab.

purchase tab

Look for OS X Mountain Lion, hold down the Option key and click Download.

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. Note that this may take up between 10 and 60 minutes, depending on your Internet connection speeds.

download mountain iion

While you wait, get a coffee, a break and a USB drive with at least 8 GB capacity. Make sure that you won’t need any of the contents of the flash drive, if you do, keep a backup of it somewhere first.

When you get back, we will look at how you can create a bootable copy of OS X Mountain Lion on your USB drive.

2. Create bootable OS X Mountain Lion

Do not start the installation process when your download finishes. Our goal is here to keep a soft copy of the installer and in the flash drive.

Once your download is done, go to Finder > Applications. Look for the file called "Install OS X Mountain Lion", right-click and select Show Package Contents.

show package content

Navigate to Contents > SharedSupport, then look for the filename "InstallESD.dmg"

mountain lion installer file

Double-click InstallESD.dmg to open/mount it on your Desktop. This may take a minute or two.

mount installer

Plug in the flash drive into your Mac and launch the Disk Utility application. You can do it quickly by searching via Spotlight.

disk utility via spotlight

In Disk Utility, do the following 4 steps carefully:

  1. Select the USB drive on the left menu,
  2. Select the Erase tab,
  3. Select Mac OS Extended (Journaled) for the format.
  4. Click the Erase button.

format usb as mac extended

Once the flash drive is formatted, re-select the formatted USB drive from the left menu, and click the Restore tab.

select restore tab

Next, we are going to create a bootable OS X Mountain Lion in the flash drive. Do the following steps carefully:

  1. Drag the formatted drive from the left menu to the Destination field.
  2. Drag "Mac OS X Install ESD" on the left menu to Source.
  3. Click the Restore button. Installation may take about 10 – 20 minutes.

restore source destination

Once it’s done you’ll have yourself a flash drive with a bootable Mountain Lion.

3. Reboot, Clean up and reinstall

So we’ve downloaded a new copy of Mountain Lion and made it bootable in a flash drive. Next up, we’ll attempt to remove everything from the current OS and reinstall a new copy of Mountain Lion.

First, a reboot! Hold down the Option key as you restart and you’ll end up at the screen below. Then, select the Mac OS X on the right.

restore source destination

You’ll be taken to the OS X Utilities dialogue. Select Disk Utility.

restore source destination

At the Disk Utility screen, make sure you have these selected:

  • You are selecting your Mac’s hard disk drive (and not your flash drive’s) on the left menu.
  • You are on the Erase tab.
  • You have selected the "Mac OS Extended (Journaled)" format.

restore source destination

Click the Erase button button and everything (including files, applications, settings, etc) in your Mac will be completely deleted.


(3.4) When you are done, you’ll be directed back to the OS X Utilities dialogue. This time, select Reinstall OS X, the second option from the list.

restore source destination

That’s it!

Your Mac will now reinstall from the flash drive and you’ll end up with a clean piece of Mac OS X Mountain Lion OS, as if your MacBook or iMac was brand new.

What’s Next?

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