How to Boot a Mac from USB Drive

With CD/DVD drives disappearing from new-age computers, USB storage is slowly turning into the only option when you look to re-install an operating system. Here we try to do it the mac way!

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   The download can take some time depending on your internet connection. Don't worry if you lose connection halfway through download, it will resume as soon as you are re-connected to App Store.
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   Make a backup of Installer downloaded from App Store and save it on the desktop. Under no condition should you work on the original installer file.
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   We will use the backup file to make bootable USB Drive.
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   Right click (Two finger click) on the installer file and click on "Show Package contents".
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   This will open a new window with contents of installer file.
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   Navigate to Contents > SharedSupport.
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   You’ll see a disk image called “InstallESD.dmg.” This is your ticket to creating a bootable copy of OSX Mountain Lion.
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   Go to search icon and type "Disk Utility".
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    Click on "Disk Utility" Application and wait for it to open.
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    Drag the “InstallESD.dmg” file from installer folder into the white box on the left side of Disk Utility, and the disk image will be added.
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    Connect your USB drive to the mac using provided slots.
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    Wait for it to be recognised and displayed on the desktop.
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    Select the USB from the list in Disk Utility.
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    Click on "Erase"
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    make sure it’s partitioned as “Mac OS Extended (Journaled)” under the “Partition” section in Disk Utility.
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    Name will be "Untitled" by default, you are free to change it as per your liking.
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    Click on "Erase" in bottom right corner.
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    Wait for USB disk to be erased and restored to single clean and usable partition.
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    Click on “InstallESD.dmg” icon on the left side of Disk Utility app.
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    Click on "Restore" tab in the centre-top part of Disk Utility App.
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    You will notice that “InstallESD.dmg” is already mentioned in source tab.
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    To start making your bootable copy of Mountain Lion, drag the drive you just added to Disk Utility from the source list in the top left of the app’s white space to the “Destination” path.
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    Hit “Restore” and let Disk Utility work its magic. This could take some time so please be patient.
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    Close Disk Utility.
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    You now have a bootable USB disk drive with Mac OSX Mountain Lion installed and good to go!
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    Restart the computer you wish to boot from this disk.
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    While restarting, hold down 'option' key.
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    Select the Installer app from the menu and you will be able to boot from it.
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    Congratulations! You can now use this disk to verify/repair disk partitions, repair current installed OSX, upgrade to new OSX, or perform a clean install of existing 

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