Because OS X Mountain Lion is a software download, it does not include a physical installer in the form of a bootable DVD or USB flash drive. Some Mac users may find it convenient to have the OS X installer on portable media, such as a DVD or flash drive, when performing a clean installation to a startup disk.
How to Create a Bootable Mac OS X Mountain Lion 10.8 USB Flash Drive
This guide will walk you through the process of creating a bootable OS X Mountain Lion installation DVD or USB flash drive.
You'll need a dual-layer DVD and a burner or a USB flash drive. The dual-layer DVD has two layers, increasing the available recording space to about 8.5 GB. The OS X Mountain Lion installer is slightly too large to fit on a standard DVD. Dual-layer DVDs are available wherever standard DVDs are sold. If your Mac doesn't have a built-in SuperDrive, use an external DVD burner.
You can also use a USB stick with a capacity of at least 5 GB as a boot medium. USB sticks of 8 GB and 16 GB are also often available.
