The Mountain Lion install image that we need to create either the bootable DVD or the bootable USB flash drive is contained within the Install OS X Mountain Lion file we downloaded from the Mac App Store.
Because the image file is contained within the downloaded file, we need to copy it to the Desktop to make creating the bootable image as easy as possible.
- Open a Finder window, and navigate to your Applications folder (/Applications).
- Scroll through the list of files and locate the one named Install OS X Mountain Lion.
- Right-click the Install OS X Mountain Lion file and select "Show Package Contents" from the pop-up menu.
- You will see a folder named Contents in the Finder window.
- Open the Contents folder, and then open the SharedSupport folder.
- You should see a file named InstallESD.dmg.
- Right-click the InstallESD.dmg file and select "Copy InstallESD.dmg" from the pop-up menu.
- Close the Finder window and return to the Desktop.
- Right-click on an empty area of the Desktop and select "Paste Item" from the pop-up menu.
Pasting the item to the Desktop can take a bit of time, so be patient.
When the process is finished, you'll have a copy of the InstallESD.dmg file that we need to create bootable copies.