How to Create a Bootable OS X Mountain Lion USB Install Key

These are instructions on how to create a bootable OS X Mountain Lion USB key.

Step One
Connect a 8GB or larger USB key to your computer.


Step Two
On a Mac that can run OS X Mountain Lion, open Apple's How to download and install macOS page in Safari and click Mountain Lion 10.8 under 'Download via web browser'. The InstallMacOSX.dmg disk image downloads to your Downloads folder. Apple no longer sells OS X Mountain Lion in the Mac App Store; this download is free.

Step Three
Double-click InstallMacOSX.dmg to open it, then double-click InstallMacOSX.pkg and follow the onscreen instructions. This puts the Install OS X Mountain Lion app in your Applications folder. If the Mac isn't compatible with OS X Mountain Lion, the package won't complete.

**If you'd rather install OS X Mountain Lion on this Mac, follow these instructions.

Step Four
If the installer opens, choose Quit Install OS X Mountain Lion from the Install OS X Mountain Lion menu to exit it.

Step Five
Click to launch a new Finder window from your dock.


Step Six
Navigate to the Applications folder and right click the new "Install OS X Mountain Lion" icon then select Show Package Contents from the contextual menu.




Step Seven
Double click the Contents folder.


Step Eight
Then double click the SharedSupport folder


Step Nine
Right click or 'Control' click InstallESD.dmg and select Disk Utility from the Open With menu.




On OS X El Capitan or later, Disk Utility has no Partition or Restore tabs. Select the USB drive, click Erase, name it 'MOUNTAINLION', choose Mac OS Extended (Journaled) and GUID Partition Map, then click Erase. Then select the new volume, choose Edit > Restore, click Image, pick the InstallESD.dmg file and click Restore.

Step Ten
Select your USB thumb drive from the list on the left and click the Partition tab.


Step Eleven
Set the Partition Layout to 1 Partition, set the Name to 'MOUNTAINLION', set the Format to Mac OS Extended (Journaled), and then click the Options button.


Step Twelve
Choose GUID Partition Table from the popup window then click the OK button.


Step Thirteen
Next click the Apply button to save your changes.


When asked to confirm click the Partition button.


Step Fourteen
Right click or control-click the InstallESD.dmg image from the left side of Disk Utility and choose Open Disk Image from the popup. Select the new Mac OS X Install ESD partition that appears then click the Restore tab.


Step Fifteen
Make sure the Mac OS X Install ESD is set as the Source then drag the MOUNTAINLION partition from the list on the left into the Destination field.


Step Sixteen
Click the Restore button then click the Erase button from the confirmation popup to begin building your install key.


Another popup will appear asking you to log in. Enter your administrative username and password then click OK.


Step Seventeen
When the restore is complete the OS X Mountain Lion USB Install Key will be mounted and it will be ready to use.