How to Erase and Reinstall Mac OS X Mountain Lion

You may anytime come across a situation that calls for erasing the startup disk on your Mac and reinstalling OS X. Further, if you are planning for an upgrade to OS X Mountain Lion, then following instructions will suggest you the recommended course of action to follow for a safe and convenient OS X upgrade:

NOTE: Before erasing and reinstalling Mac OS X, you should back up all your important data to an external media device. If something interrupts the process, you can rest assured that your data will not be affected.

Erasing Mac OS X Startup Disk

• Navigate to the Apple menu and click 'Restart'. As your Mac restarts, continuously press the 'Command' and 'R' keys on your keyboard.
• You can connect to the Internet by selecting a Wi-Fi network from the Wi-Fi menu (In case you are not already connected).
• Choose Disk Utility and click 'Continue'.
• Choose your Startup Disk from the left panel of the Disk Utility window, and then select the 'Erase' tab in the top-right corner of the screen.
• On the format pop-up menu, select 'Mac OS Extended (Journaled)' and specify a desired name for the startup disk. Next, click 'Erase'.
• Once the process is finished, select Disk Utility → Quit Disk Utility.

Reinstalling OS X Mountain Lion

NOTE: For reinstalling OS X, you must be connected to the Internet. 

• Once you quit Disk Utility, choose 'Reinstall OS X'. Next, click 'Continue'.
• Follow the step-by-step instructions displayed on the screen. In the pane that asks you to choose a disk, choose the current startup disk.
• In order to initiate the installation process, click 'Install'.

You can use the Recovery Disk Assistant (a feature exclusive to OS X Lion and Mountain Lion) to create a 'Recovery HD' partition on any external Mac hard drive. It will provide you the same set of capabilities as the 'Recovery' built into your Mac OS X Lion or OS X Mountain Lion.

All the data on your external drive will be wiped out after running the Recovery Disk Assistant. Before doing this, you should back up all essential files to another drive. You can also choose to create a new partition and then run the Recovery Disk Assistant to install 'Recovery HD'. You will not be able to see this partition either in the Finder or in Disk Utility. In order to use 'Recovery HD' installed on an external drive, you should attach the drive to your Mac, reboot the system, and keep pressing the 'Options' key. When the Startup Manager launches, choose 'Recovery HD'.