Create a Mac OS X Mountain Lion Bootable USB Drive

Create a bootable Mac OS X Mountain Lion (10.8) installer on a USB drive by following these six easy steps, three of which you've probably already completed! Optionally, follow step 7 to install Mountain Lion on your computer using the USB drive created in step 6.

1 Get a supported Mac computer

You must have a Mac computer to run OS X Mountain Lion. The following models are supported:

  • iMac (Mid 2007 or newer)
  • MacBook (Late 2008 Aluminum, or Early 2009 or newer)
  • MacBook Pro (Mid/Late 2007 or newer)
  • MacBook Air (Late 2008 or newer)
  • Mac mini (Early 2009 or newer)
  • Mac Pro (Early 2008 or newer)
  • Xserve (Early 2009)

If you do not have one of these models, you can purchase a Mac from the Apple Store. You can also search Amazon for Apple computers in the box below.

Note: if you purchase a new Mac today, Mountain Lion will be installed for you. If that meets your needs, you can stop reading this guide now. If you would like to create a USB installer, please continue.

Your Mac must be running Mac OS X Snow Leopard (10.6), Mac OS X Lion (10.7), or Mac OS X Mountain Lion (10.8).

If you are running Snow Leopard, first run Software Update to ensure you are running version 10.6.8, the latest version.

Mac OS X Software Update

Go to Step 2

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2 Get a USB drive

A USB drive is also sometimes called a flash drive or thumb drive. An 8 GB model would be a suitable choice. A model with more space is fine. The minimum amount of space required is 8 GB.

If you do not have one, purchase an 8 GB USB drive from Amazon. You can also search Amazon for USB drives in the box below.

Go to Step 3

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3 Purchase Mac OS X Mountain Lion (10.8) from the App Store

Open Mac OS X Mountain Lion in the Mac App Store. The App Store application on your computer will launch. Purchase the OS X Mountain Lion software by clicking the gray button under the picture of the mountain lion.

Mac OS X Mountain Lion in the Mac App Store

Alternatively, open OS X Mountain Lion in the iTunes website. On that page, click the blue button to View in Mac App Store and then continue the purchase as above.


After you purchase the software, the program will download to your computer. When the download is complete, the installer program will launch automatically. Do not install the program at this time. Quit the installer program.

Quit Install Mac OS X

Go to Step 4

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4 Download the Mac OS X Mountain Lion (10.8) installer from the App Store

Even after you purchase the software, it is possible the installer is not stored permanently on your hard drive, hence the reason for this extra step.

Go to your Applications folder. Look for the Install OS X Mountain Lion application.

Locate the Install OS X Mountain Lion application

If the file is there, go to Step 5. If you do not see it, please continue.

Open the App Store application and select Purchases at the top of the window. Find OS X Mountain Lion in the list of purchases. If you do not see it, go back to Step 3 and purchase the software.

On the right side of the Purchases window, click the Download button.

If you are already running OS X 10.8, you will see a warning that says 10.8 is already installed. Click Continue.

Download the Mac OS X Mountain Lion installer from the App Store

Just as in Step 3, if the installation program launches after the download is done, quit the installation. Do not install Mountain Lion at this time.

Quit Install Mac OS X

Go to Step 5

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5 Locate the installer disk image

Open the Disk Utility program. It is located in Applications > Utilities.

In the left pane of the Disk Utility window you will see a list of disks connected to your computer. Look for the compressed disk image file called InstallESD.dmg. Indented underneath it should be the disk image labeled Mac OS X Install ESD.

Look for InstallESD.dmg

If you see the indented disk image, go to Step 6. If it is not there, please continue.

Go to your Applications folder. Look for the Install OS X Mountain Lion application.

If you do not see it, go back to Step 4 to download the installer. If it is there, Right-click the file and select Show Package Contents.

Show Package Contents

Open the Contents folder. Open the SharedSupport folder. Locate the file called InstallESD.dmg. Double-click the file to open and mount it on the Desktop.

Locate the InstallESD.dmg disk image file

A new Finder window will open, displaying the contents of the disk image. You can close that window and the SharedSupport window.

Go to Step 6

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6 Create the bootable USB drive

Plug in the USB drive now. If there are any files on the USB drive that you want to save, make a backup of those files now.

Open Disk Utility if it is not already open. Select Mac OS X Install ESD in the left pane. In the right pane, click the Restore tab.

Select Mac OS X Install ESD from the left pane and click Restore

From the left pane, find the USB drive you plugged in. Find the disk partition indented underneath the USB drive. Click and drag the icon for the disk partition over to the Destination input box in the right pane. When the cursor changes to one with a green plus sign, drop the icon.

Drag the USB drive partition to the Destination input box

Click the Restore button in the bottom right corner. You will see a dialog box asking if you want to erase the USB drive. Click Erase. You may also have to enter your user account credentials.

The OS X Mountain Lion installer will now be copied to the USB drive. When the process is complete, you are done.

Congratulations! You now have a USB drive that you can use to install Mountain Lion on any of the Mac computers you own.

If you would like to follow the final step to install Mountain Lion on your computer, go to Step 7.

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7 Install Mountain Lion on your computer

This step is optional. You do not have to install Mountain Lion on your computer if you are not ready. You have a working, bootable USB drive and you can install the new OS at any time. Before you continue, we invite you to read the words of caution below.

Caution! Important! Turn back!

Before you upgrade to Mountain Lion, it is a good idea to make a complete backup of your computer. You can use your Mac's Time Machine application to create a backup using an external hard drive of your choice.

There are also a number of third party applications that can create backups to an external drive. Additionally, there are several cloud drive services that use the web to transfer important files to an offsite data center.

Please ensure that the files you wish to keep are properly backed up before continuing. To achieve the best performance on your computer, it is recommended, though not required, that you erase your computer's main hard drive when you install Mountain Lion.

As a result of erasing your hard drive, after you install Mountain Lion you will have to transfer your important files back to your computer and re-install any programs that do not come pre-installed with Mountain Lion. For any software you use that allows you to customize settings, you will also have to customize the settings again, including any settings made to OS X.

As an example of what to back up, you may want to have:

  • a list of all software you want to re-install and any license information required to install them,
  • a list of any settings used to set up and customize your software,
  • a list of all account information and passwords used for any software that may require it,
  • a list of all account information and passwords used for any websites of which you are a member,
  • a backup and list of crucial files that you keep on your computer and their external storage locations, such as wills, tax files, financial files, et cetera,
  • a backup and list of files and settings used for work, such as office documents, VPN or other server data, if any,
  • a backup of any multimedia files, like photos, videos, and music,
  • and a backup of any other files and settings you wish to keep that do not fall into the above bullet points

The checklist above is merely suggestive and may be far from exhaustive depending on your computer usage. Making a proper backup of your computer is beyond the scope of this guide.

If the process of backing up and re-installing all software and settings seems daunting, or if you are unsure about the thoroughness and reliability of your backup, you should consider simply upgrading OS X in place without erasing your hard drive. The installation process makes it very easy to upgrade your operating system without disturbing your current applications and files. Indeed, this is what most people do. You should still make a complete backup of your computer, but during the Mountain Lion installation process you will leave your hard drive alone and choose to import your files and settings from the backup rather than erasing and starting over from scratch.

If you continue to follow this guide, we walk through the steps necessary to get to the installation wizard. If you do not wish to erase your hard drive, it is possible to skip that step.

When you are ready to install, please go to Step 7a, Rebooting.

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7a Rebooting

If your bootable USB drive is not plugged in, do that now. Also, make sure you have a way to view this guide without using your computer. You may wish to print the guide or visit this website on a mobile device such as an iPhone or iPad.

Restart your computer. As it restarts, hold down the Option key. When it is done rebooting you will see a list of the disks connected to your computer, including your main hard drive and the USB drive you just created. You can remove your finger from the keyboard now. Double-click the USB drive icon or click the arrow underneath it to boot your computer from that drive.

Select the USB drive as the boot disk

The installer disk will now load. When it is finished you will see a window with various actions you can take. The two that we care about are Disk Utility and the Install OS X application.

The OS X Boot Disk Utilities

If you have made a backup of your computer and you are willing to re-install everything (see the warning above), we highly recommend erasing your main hard drive before installing Mountain Lion as this will provide the best performance for your computer. This is not required. You can still have a nice, comfortable life enjoying the benefits of Mountain Lion without the additional benefits of starting from scratch. It's just that we find starting from scratch so much more satisfying. Not to mention, your computer will perform like new again.

If you would like to erase your drive, continue with 7b, Erasing your hard drive. Otherwise, skip over that section and go straight to 7c, Installing Mountain Lion.

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7b Erasing your hard drive

Select Disk Utility from the menu and press the Continue button.

After it loads, the left pane will contain a list of drives connected to your computer. Your main hard drive is most likely the one at the very top of the list. Please confirm that you are selecting the correct disk. Your main disk contains a partition--a section of the disk--that holds the contents of your computer, including your applications and files. The name of the partition will be indented under the name of your main hard drive. Do not select the partition. Select the main hard drive above it.

Select your hard drive in Disk Utility

Click the Erase tab. Then click the Erase button at the bottom far right of the window. When it is done erasing, please quit the Disk Utility application.

Go to Step 7c

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7c Installing Mountain Lion

Finally, the moment of truth.

Select Reinstall OS X from the menu and press the Continue button. Follow the instructions in the installation wizard to install Mountain Lion.

Install Mountain Lion

Congratulations! You are done. Please enjoy your new operating system.

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