How to create a bootable OS X 10.8 Mountain Lion installer from any USB drive, flash key or external hard drive.

By creating a bootable install drive, you can perform clean OS X 10.8 installs, install it on separate partitions, and install OS X Mountain Lion on Macs that don’t have internet connections.

If you already have the Mac OS X Mountain Lion install DMG extracted, skip this first set of steps and jump directly to making the bootable drive below.

Download OS X Mountain Lion and Extract the DMG File

Download OS X 10.8 Mountain Lion from the App Store

Go to the /Applications/ directory and right-click on “Install Mac OS X Mountain” selecting “Show Package Contents”

Open the “Contents” directory and then open “SharedSupport”, looking for a file named “InstallESD.dmg”

Double-click on InstallESD.dmg to mount it on the desktop

Make the OS X Mountain Lion Install Drive

Launch Disk utility and connect a USB drive to the Mac

Select the USB drive from the left menu and click on “Erase” tab, choose “Mac OS X Extended (Journaled)” as the
format, then click on the “Erase” button in the corner

Now select the formatted USB drive from the left side and click on the “Restore” tab

Drag the previously mounted “Mac OS X Install ESD” image into the “Source” section

Drag the formatted partition to the “Destination” section, then click on “Restore”

Confirm that the USB drive will lose it’s data and enter the Admin password when asked
Disk Utility will now create a bootable OS X Mountain Lion installer drive out of USB drive using the disk image, this can take a little while depending on how fast the drive and Mac are but 20-30 minutes isn’t unusual.

When finished, reboot the Mac and hold down the Option key to bring up the boot menu:

Select the orange “Mac OS X” option and you will boot into the OS X Mountain Lion installer, from here installation is the same as normal. Click on Continue, select the destination drive, and install.

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