Boot a Mac from USB Drive

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CD/DVD drives are disappearing from computers, leaving USB storage as the only option when re-installing an operating system. The process is fairly painless and, if you have a bit of time and patience you can complete the task on a Mac.

Steps

  1. Download Mac Lion Installer from App Store. Latest versions of Mac are only available through App Store.
  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.
  3. Make a backup of the installer you downloaded from App Store and save it on your desktop. Under no condition should you work on the original installer file.
  4. Use the backup file to make a bootable USB drive.
  5. Right click (Two finger click) on the installer file and click on "Show Package contents".
  6. This will open a new window with contents of installer file.
  7. Navigate to Contents > SharedSupport.
  8. You’ll see a disk image called “InstallESD.dmg.” This is your ticket to creating a bootable copy of OSX Mountain Lion.
  9. Go to search icon and type "Disk Utility".
  10. Click on "Disk Utility" Application and wait for it to open.
  11. 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.
  12. Connect your USB drive to the Mac using the provided slots.
  13. Wait for it to be recognized and displayed on the desktop.
  14. Select the USB from the list in Disk Utility.
  15. Click on "Erase."
  16. Ensure it’s partitioned as “Mac OS Extended (Journaled)” under the “Partition” section in Disk Utility.
  17. The name will be "Untitled" by default, you are free to change it as per your liking.
  18. Click on "Erase" in bottom right corner.
  19. Wait for USB disk to be erased and restored to single clean and usable partition.
  20. Click on “InstallESD.dmg” icon on the left side of Disk Utility app.
  21. Click on "Restore" tab in the centre-top part of Disk Utility App. “InstallESD.dmg” should already be present in source tab.
  22. 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.
  23. Hit “Restore” and let Disk Utility work its magic. This could take some time so please be patient.
  24. Close Disk Utility. You now have a bootable USB disk drive with Mac OSX Mountain Lion installed and good to go!
  25. Restart the computer you wish to boot from this disk.
  26. While restarting, hold down 'option' key.
  27. Select the Installer app from the menu and you will be able to boot from it.
  28. 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 or better OSX.

Tips

  • Always insist on original software as supplied by Apple Inc. and downloaded from Mac App Store.
  • There are some third party apps like "Lion Disk Maker" which specialize in creating bootable disk for Mac.

Warnings

  • Do not disconnect USB drive in middle of entire process.
  • Always keep a backup of your installer file before copying to USB/external drive.
  • A bootable Mac disk will work on Mac Computers ONLY.
  • Always backup your system using Time Machine before installing any new OSX.

Things You'll Need

  • 8GB+ USB external drive
  • Mac OSX Installer from App Store
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