“It’s a good idea to have a bootable emergency drive on hand, just in case disaster strikes your Mac,” Matt Cone writes for Macinstruct.
“An emergency drive (also referred to as an OS X Recovery Disk) can help you repair the hard disk, reinstall the operating system, and restore from a Time Machine backup to get your computer back fast,” Cone writes. “With previous versions of OS X, you could have used the installation DVD to fix problems. But OS X Mountain Lion is sold in the App Store as a digital download — no physical disk is provided. What’s a maintenance-minded Mac user to do?”
Cone writes, “Create your own bootable OS X USB drive, of course! It’s easy, and if you’ve already purchased OS X and have a USB drive that’s 1 GB or larger, it’s completely free. Carry it in your ￼￼￼￼￼￼￼￼￼￼pocket or put it on your keychain so it’s available if the worst-case scenario occurs. You’ll thank yourself for taking the time to complete this project.”
Complete instructions in the full article here.