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Summary:

Perhaps you have a fast broadband connection, but there are times when it’s more useful to have a means to install OS X from an external source. Here’s a quick guide to creating a USB installer for OS X Mountain Lion.

Apple-Mountain-Lion

Perhaps you have a fast broadband connection at home or work and downloading multiple copies of OS X Mountain Lion from the Mac App Store to update a few computers is not an issue.  But there are times when it’s more useful to have a means to install OS X from an external source.  The following is a quick guide to help you create a USB installer for Mountain Lion, very similar in fact to the OS X Lion USB Thumb Drive Apple use to sell with previous versions of OS X.

Step One - Extract the Drive Image

Step 1: Extract the drive image

After purchasing and downloading the OS X Mountain Lion update and before actually updating your Mac, navigate to your Applications folder in the Finder and right click on the file “Install Mac OS X Mountain Lion.app” and select “Show Package Contents.”  Look in the “Shared Support” folder located under the “Contents” folder of the “Install Mac OS X Mountain Lion.app,” and copy the file named “InstallESD.dmg” to your desktop.  This file is a bootable disk image of OS X with the Mountain Lion Installer already installed.  You will need this file in step three.

Step Two - Erase a USB Drive

Step 2: Erase a USB drive

Take a thumb drive that has at least a 8GB capacity, and connect it to your Mac. Launch the Disk Utility application located in your Application’s Utilities folder.  Once it is running you will see a list of the mounted drives connected to your Mac. Make sure there are no files you need on the drive, then select the USB drive, and click on the Erase tab to the right.  Select Mac OS Extended (Journaled) as the Format, set the Name to “Mac OS X Installer” and click on the Erase button.

Step Three - Restore the Drive Image

Step 3: Restore the drive image

Now select the “Mac OS X Installer” you created in step two, and click on the Restore tab to the right.  You want to set the Source as the “Mac OS X Installer ESD,” which is located within the “InstallESD.dmg” file you copied in step one.  Then set the Destination to be the “Mac OS X Installer” partition you created in step two.  You can drag and drop the items in the list on the left of the Disk Utility, into their appropriate Source and Destination settings on the right.  If you do not see the “InstallESD.dmg” file listed, use the Image button and locate it on your desktop.  Click on the Restore button once everything is set correctly.

And that’s it.  Once the process is complete, you will have your own backup installer for OS X Mountain Lion that you can use to install on your Mac, or restore it should something go terribly wrong.  When you boot your Mac, be sure that the USB thumb drive is inserted, and hold down the option-key when the Mac starts up.  You will be presented with a list of drives.  Simply select the “Mac OS X Installer” you just created on your USB drive and you will be ready to install OS X Mountain Lion on your Mac.

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  1. What’s the minimum size USB stick that you need to do this? 8Gb?

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    1. The Lion USB Stick from Apple had a capacity of 4.19 GB and used 3.82 GB of available space. I was able to create a 5 GB Mountain Lion Install partition on a 16 GB USB Thumb Drive. The Mountain Lion Installer Partition took up 4.38 GB of space. So yes, Mountain Lion should fit on a 8 GB USB Thumb Drive.

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      1. Can you fit additional information on the USB Thumb drive (such as a Time Machine backup) or is the entire USB Stick closed?

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  2. Steve, no, you cannot use the drive for anything else

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    1. Andrea Parisi Tuesday, August 7, 2012

      @Chris, why can’t you store other files?
      The usb drive works like a normal usb drive that contains stratup files for OSX installation.
      I did it with OSX Lion. I think it’s possible also with OSX Mountain Lion.

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  3. Hello GEOFFREY, I have trouble in opening my mac. The OS X is missing I guess, I am only seeing the OS X UTILITIES menu and nothing more, so what I did, I downloaded OS X Mt. Lion dmg (from Torrent), is there a way that i can install this? Please advise, is so pls provide me the steps, I am a newbie to mac..Thank

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  4. Hey Wyred, I’m having the same problem with you

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  5. Same issue as the guys above.

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  6. how can i do this for snow

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  7. Just to be 100% sure: this technique won’t erase startup disk? it’s just an upgrade, right? I don’t need any backup, right?

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    1. This technique is to create a USB installer. The reason you would want to do such a thing is to re-install Mountain Lin on a drive that has been erased (what is know as a clean install).

      If you have never used the Disk Utility application before, then you should be fully backed up before doing so. Any time you are working with Disk Utilities and Operating System installers is a good time to back up everything you’ve got.

      As a general rule, if you have to ask, should I perform a backup before doing this, then you should perform a backup before doing this.

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