Long gone are the days where Apple sells Mac OS X on discs. To get Mountain Lion (Mac OS X 10.8) you buy it online from Mac App Store, download and install it. That’s great as long as your Mac is working properly and online. As we all know there are times when you may need to reinstall Mac OS X, install it on a fresh system or external hard drive. When you install it on a Mac you need to be able to boot from an external disc especially if you want to reformat the hard drive or do repairs with Disk Utility. Apple used to make it kinda easy to create a bootable disc, but for some reason with Mountain Lion Apple no longer provides to utility to automate this process. Nevertheless it can still be done.
What you need
You’re gonna need the Mountain Lion installer. You may need to redownload it from the Mac App Store:
You’re also gonna need a USB thumb/flash drive. Unfortunately Mountain Lion takes a bit more than 4GB, so you’re gonna need at least an 8GB Thumb Drive. Since my MacBook Pro and MacBook Air have USB 3.0 ports I decided to go with USB 3.0 Thumb Drives to speed up the process and future installations. You can get them here from Micro Center.
Note: Not all Macs can boot from USB 3 Flash Drives/Not all USB 3 Flash Drives are bootable. If you have a Mac with only USB 2 ports, its’ probably safer to go with a USB 2 Flash Drive.
How to Create a Bootable Mountain Lion USB Flash Drive
Step 1. Open your Applications folder (or where ever you moved/copied the Mountain Lion installer “Install OS X Mountain Lion”. Right click on it and choose Show Package Contents.
Step 2. Open the Contents folder, then SharedSupport Folder and then double click on the InstallESD.dmg to mount it.
Step 3. Plug in you 8GB or larger USB Flash Drive.
Step 4. Launch the Disk Utility located in your Utilities folder in your Applications folder.
Step 5. Erase your USB Flash Drive to format it as a Mac disc.
Step 6. Using the Restore tab, restore the Install image you mounted in step 2 to your newly reformatted USB Flash drive.
Step 7. Once the Restore completes you can quit Disk Utility and then unmount your USB Flash Drive.
Now whenever you need to boot from your USB Flash Drive just plug it in, Restart your Mac while holding down the Option Key. You should then be able to choose your USB flash drive to boot from. Once booted you will be able to install Mountain Lion or use Disk Utility to repair/format your drive.