Installing Mountain Lion Clean

Mountain Lion

Since the unveiling of OS X 10.8, people have been asking me if Mountain Lion, which will be distributed exclusively over the Mac App Store this summer, can be burned to a DVD using the same procedure I outlined "Burning a Lion Boot Disc." The answer is yes. Both the Lion and Mountain Lion installers contain a full Mac OS X installation image suitable for booting a compatible Mac and performing a clean install. All you have to know is how to get at the image.

  1. Purchase and download Mountain Lion from the Mac App Store on any 10.8 compatible Mac running 10.6 or 10.7.
  2. Right click on "Install Mac OS X Mountain Lion.app" and choose the option to "Show Package Contents."
  3. Inside the Contents folder that appears you will find a SharedSupport folder and inside the SharedSupport folder you will find the "InstallESD.dmg." This is the Mountain Lion boot disc image you are looking for.
  4. Copy "InstallESD.dmg" to another folder like the Desktop.
  5. Launch Disk Utility and click the burn button.
  6. Select the copied "InstallESD.dmg" as the image to burn, insert a dual-layer DVD (at least 4.76 GBs in capacity), and wait for your new Mountain Lion Boot Disc to come out toasty hot.

If you want to prepare a bootable external USB drive capable of installing OS X 10.8 follow these instructions.

(For best results you want to use an external drive formatted with a GUID partition scheme. From Disk Utility select the drive you wish to use in the left hand source pane, and choose the partition tab. The options button allows you to specify the GUID partition scheme. Click Ok, and then Apply to reformat your drive.)

  1. Purchase and download Mountain Lion from the Mac App Store on any 10.8 compatible Mac running 10.6 or 10.7.
  2. Right click on "Install Mac OS X Mountain Lion.app" and choose the option to "Show Package Contents."
  3. Inside the Contents folder that appears you will find a SharedSupport folder and inside the SharedSupport folder you will find the "InstallESD.dmg." This is the Mountain Lion boot disc image you are looking for.
  4. Launch Disk Utility and go straight to the Restore tab.
  5. Drag "InstallESD.dmg" the Source field, and drag your 8GB or larger external hard drive from the left side of the Disk Utility window into the Destination field. [1]
  6. Check Erase Destination (if present), and begin your restore. (It goes without saying this process will erase everything on your external hard drive.) Once Disk Utility's restoration is complete you will be the proud owner of a external drive capable of booting and installing Mountain Lion clean.

Once you have created your install media follow these steps to install Mountain Lion clean.

  1. Insert your installation DVD or connect your external boot drive into any 10.8 compatible Mac.
  2. Restart the Mac holding the option key, and choose Mac OS Install ESD as the startup volume.
  3. Once booted from the Mountain Lion installation media you will have the opportunity to run Disk Utility and erase your Mac's primary hard drive. From the Erase tab select your Mac's primary hard drive and choose "Mac OS Extended (Journaled)" as the format. Before you erase the drive confirm you have a known good backup then click Erase.
  4. With your Mac's primary hard drive wiped clean it is now time to begin installing Mountain Lion. Quit Disk Utility to go back to the Mountain Lion installer. Choose your Mac's primary hard drive as the destination, and continue.

Call me old fashioned, but I always format my Mac's hard drive and perform a clean install when a new version of Mac OS X is released. The arrival of Mountain Lion this Summer will be no different.

It is important that you burn your Mountain Lion boot disc or backup the Mountain Lion installer prior to installing 10.8. If left in the Applications folder the installer will be removed after the Mountain Lion installation is complete. If you are reading this article after upgrading to Mountain Lion all is not lost. A fresh copy of the Mountain Lion installer can always be re downloaded from the Mac App Store by clicking on the Purchased tab with the Option key held down.


  1. Snow Leopard users will want to mount the "InstallESD.dmg" prior resotring and drag the mounted "Mac OS X Install ESD" into the Source field.  ↩