How to Boot a Mac from USB Drive

Step-by-Step Guide

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   Step 1: Download Mac Lion Installer from App Store.

   Latest versions of Mac are only available through App Store.
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   Step 2: 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. , Under no condition should you work on the original installer file. ,,,,,,,,,,,, ,,,,,, “InstallESD.dmg” should already be present in source tab. ,, This could take some time so please be patient. , You now have a bootable USB disk drive with Mac OSX Mountain Lion installed and good to go! ,,,, 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 or better OSX.
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   Step 3: Make a backup of the installer you downloaded from App Store and save it on your desktop.

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   Step 4: Use the backup file to make a bootable USB drive.

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   Step 5: Right click (Two finger click) on the installer file and click on "Show Package contents".

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   Step 6: This will open a new window with contents of installer file.

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   Step 7: Navigate to Contents > SharedSupport.

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   Step 8: 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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   Step 9: Go to search icon and type "Disk Utility".

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    Step 10: Click on "Disk Utility" Application and wait for it to open.

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    Step 11: Drag the “InstallESD.dmg” file from installer folder into the white box on the left side of Disk Utility

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    Step 12: and the disk image will be added.

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    Step 13: Connect your USB drive to the Mac using the provided slots.

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    Step 14: Wait for it to be recognized and displayed on the desktop.

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    Step 15: Select the USB from the list in Disk Utility.

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    Step 16: Click on "Erase."

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    Step 17: Ensure it’s partitioned as “Mac OS Extended (Journaled)” under the “Partition” section in Disk Utility.

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    Step 18: The name will be "Untitled" by default

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    Step 19: you are free to change it as per your liking.

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    Step 20: Click on "Erase" in bottom right corner.

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    Step 21: Wait for USB disk to be erased and restored to single clean and usable partition.

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    Step 22: Click on “InstallESD.dmg” icon on the left side of Disk Utility app.

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    Step 23: Click on "Restore" tab in the centre-top part of Disk Utility App.

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    Step 24: Drag the drive you just added to Disk Utility from the source list in the top left of the app white space to the “Destination” path.

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    Step 25: Hit “Restore” and let Disk Utility work its magic.

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    Step 26: Close Disk Utility.

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    Step 27: Restart the computer you wish to boot from this disk.

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    Step 28: While restarting

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    Step 29: hold down 'option' key.

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    Step 30: Select the Installer app from the menu and you will be able to boot from it.

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    Step 31: Congratulations!

Detailed Guide

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