Open Disk Utility (in the Applications > Utilities folder) and drag the InstallESD.dmg image to the device sidebar, where it should appear under your various disk and volumes listings.In the new window that opens, go to the Contents > SharedSupport folder and locate the file called 'InstallESD.dmg.'.Right-click the 'Install Mac OS X Mountain Lion' application in the Applications folder, and choose 'Show package contents.'.In this case, you can go to the Purchases section of the App Store and redownload the installer. If you have already installed Mountain Lion, the installer will have been automatically deleted from your system. Locate and right-click the Mountain Lion installer to show its contents (click for larger view). Quit the installer (press Command-Q) and go to the Application folder on your system. Purchase and download Lion from the Mac App Store (or use your up-to-date redemption code), and when the download is finished, the Mountain Lion installer will load.Therefore, to create a standalone USB or optical media install drive for OS X 10.8 Mountain Lion, do this: However, when following the procedure for Lion you might run into a snag that will prevent the drive from being created. We previously outlined how to create your own OS X install drive when OS X Lion was released, and the process for Mountain Lion is for the most part the same. Additionally, while Apple's OS should be available using Apple's various Internet recovery options, this can often take a long time to download, especially if you do not have the fastest broadband connections available to you. As with OS X Lion, Apple will likely release USB drives containing a bootable Mountain Lion installer, but this will cost you more than the App Store download. You also might be interested in creating a standalone install drive for Mountain Lion.

If you're interested in the new OS, then be sure to read our guide on preparing your Mac for installing the upgrade, which includes information on the OS requirements, backing up, and clearing up resolvable issues before performing the upgrade. Apple has released its OS X 10.8 Mountain Lion (see CNET's full review) and it's available for download from the Mac App Store on systems running OS X 10.6.6 or later.

Mac os 10.8 installer