I read a great article on The Unofficial Apple Weblog [TUAW] about making a Mountain Lion installer on USB.
To make an installer version of Mountain Lion on USB you’ll obviously need a copy of the OSX Mountain Lion installer.
Next, go to your applications folder and find the actual installer…should be called Mac OS X Mount Lion.
Rich click that bad boy and select the Show Package Contents form the pop-up.
Navigate to Contents > SharedSupport and then start looking for the InstallESD.dmg. This is the image file for the installer.
Now we need to launch the DiskUtility so open a new folder with Command N.
Attach your beloved USB flashdrive and be sure it’s at least 8GBs big.
Now select the USB drive and click the Partition tab and choose Partition Layout > 1 Partition.
Next, choose Format and Mac OS Extended.
Click the options button at the bottom…should be right side…and select GUID Partition Table.
So we are getting close, now click Apply and the utility will ask you to confirm…of course, confirm. Click Partition. Wait a little while the OS remounts your USB.
We are almost done making your Mountain Lion installer on USB.
Click the newly remounted partition and click the Restore tab and then click Install next to the source field.
Once that is done, mount the InstallESD.img file from before…you can do this by double-clicking the file AND DRAG the mounted image from your Mac desktop into the source location…or Drag Untitled 1 from the left column to the destination field.
Click Restore and agree to Erase the drive and replace with the contents of the .dmg file.
After you wait a while as the contents is being written to your 8GB flashdrive, you are done making your Mountain Lion installer USB.
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