Mountain Lion is here as as you already know it is a digital download available in the Mac App Store. It is always a great option to make a bootable copy of the OS and you can make one on a USB that you can carry around.
I recommend you get one at least on of 8Gb in size for the sole purpose of writing the Installer, nothing more. Follow the following steps to successfully create a bootable USB drive with Mountain Lion.
- Download OS X Mountain Lion from the Mac App Store
- Go to the Applications folder and find the OS X Mountain Lion installer and right-click on it. Then select "Show Package Contents", then select "Contents" and "SharedSupport" and find the file called "InstallESD.dmg". Double click on it to mount and you will see the image “Mac OS X Install ESD”
- No you will need to format the USB if you haven't done so:
- 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
When it is time to boot from the USB and install Mountain Lion, restart your Mac and hold the Option key down when you hear the startup chime.