Install or Reinstall Lion/Mountain Lion from Scratch
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Erase the hard drive:
1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your startup volume (usually Macintosh HD) from the
left side list. Click on the Erase tab in the DU main window.
3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on
the Security button and set the ZeroData option to one-pass. Click on
the Erase button and wait until the process has completed.
4. Quit DU and return to the main menu.
Reinstall Lion: Select Reinstall Lion/Mountain Lion and click on the Install button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
because it is three times faster than wireless.
Internet recovery is your only option unless you saved a Mountain Lion installer or have a cloned backup of the computer when it was new or created a Mountain Lion bootable flash drive from the installer you downloaded (if you downloaded one.) If Lion or Mountain Lion came with your computer then you would need to purchase and download Mountain Lion separately from the App Store.
Make Your Own Mountain/Lion Installer
1. After downloading Mountain/Lion you must first save the Install Mac OS X Mountain/
Lion application. After Mountain/Lion downloads DO NOT click on the Install button.
Go to your Applications folder and make a copy of the Mountain/Lion installer. Move
the copy into your Downloads folder. Now you can click on the Install button. You
must do this because the installer deletes itself automatically when it finishes
installing.
2. Get a USB flash drive that is at least 8 GBs. Prep this flash drive as follows:
a. Open Disk Utility in your Utilities folder.
b. After DU loads select your flash drive (this is the entry with the mfgr.'s ID and size) from the left
side list. Click on the Partition tab in the DU main window.
c. Under the Volume Scheme heading set the number of partitions from the drop down menu to one.
Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the
partition scheme to GUID then click on the OK button. Click on the Partition button and wait until
the process has completed.
d. Select the volume you just created (this is the sub-entry under the drive entry) from the left side
list. Click on the Erase tab in the DU main window.
e. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, check the
button for Zero Data and click on OK to return to the Erase window.
f. Click on the Erase button. The format process can take up to an hour depending upon the flash
drive size.
3. Locate the saved Mountain/Lion installer in your Downloads folder. CTRL- or RIGHT-click on the installer and select Show Package Contents from the contextual menu. Double-click on the Contents folder to open it. Double-click on the SharedSupport folder. In this folder you will see a disc image named InstallESD.dmg.
4. Plug in your freshly prepared USB flash drive. You are going to clone the content of the InstallESD.dmg disc image to the flash drive as follows:
a. Double-click on the InstallESD.dmg file to mount it on your Desktop.
b. Open Disk Utility.
c. Select the USB flash drive from the left side list.
d. Click on the Restore tab in the DU main window.
e. Select the USB flash drive volume from the left side list and drag it to the Destination entry field.
f. Drag the mounted disc icon from the Desktop into the Source entry field.
g. Double-check you got it right, then click on the Restore button.
When the clone is completed you have a fully bootable installer that you can use without having to re-download Mountain/Lion.
Note: The term Mountain/Lion used above means Lion or Mountain Lion.
