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.
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.