Find or Create a Startup Disk in Mountain Lion
When you're booting, you are using a particular disk or disk partition as your startup disk. You probably have a copy of the ultimate startup disk right there on your computer table — the installation DVD (or the first disc if you received more than one) that came with your computer.
For the entire history of OS X, users have kept their OS X installation disc close at hand . . . the one that came in the boxed retail copy they bought. For Mountain Lion, Apple has announced that only App Store purchases will be available for people upgrading to Mountain Lion.
In other words, unless Apple changes its policy between now and the time you read this, you won’t have a Mountain Lion Install DVD from which to work. So whatever else you do, make backup copies of the Mountain Lion Installer file that you receive from the App Store.
The lack of a bootable installer disc solution should do wonders for the sale of external hard drives and high-capacity flash drives.
With Mountain Lion, because there is no bootable DVD, the Installer creates a bootable partition named Recovery HD on that disk when you first install Mountain Lion.
Create a bootable recovery disk. The Recovery HD partition is a good concept, but if your hard disk dies, the Recovery HD partition dies too. Make a bootable clone of your startup disk as soon as possible, just in case.
Apple offers a free program called Recovery Disk Assistant, which can create a bootable Mountain Lion installer disk for you. Another option is Carbon Copy Cloner, a donationware app that lets you create a clone of your boot disk with a minimum of fuss. Or try SuperDuper! (shareware), Just add a hard disk as large as or larger than your boot disk, and you’ll be good to go.