Some issues on your Mac can be resolved by reinstalling the operating system. In the past, all Mac desktops and portables came with a dedicated recovery disc for that machine, but Macs shipping with 10.7 (OS X Lion) or later do not come with recovery discs.
These newer machines have the recovery files on a separate partition on the hard drive. Knowing these steps can come in handy if you ever need to reinstall the operating system to resolve a software problem or upgrade to a larger drive. It is also useful if you want to wipe your data and install a clean version of the Mac OS before selling or giving away your machine.
There are two types of installs. You can do a clean install, which will format the hard drive and erase all data, and then install a clean version of the OS. The other option is an OS reinstall which installs just the important OS files and leaves the user data intact.
__*Before doing either of these, it is important to back up all important data due to the possibility of failure during the installation process that can compromise your data.*__
These steps only apply to machines running 10.7 (OS X Lion) or 10.8 (OS X Mountain Lion), and require a high-speed internet connection. The actual OS files are located on Apple’s servers, and are downloaded every time you install.
*Warning*: Make sure all important data is backed up before proceeding.
1. While the machine is shut down, power the machine on and hold down the *Option* key. Then select *Recovery HD.*
2. After selecting the*Recovery HD* it will boot to the Mac OS Utilities window.
* If you want to Install the OS and keep the user data intact, select *Reinstall Mac OS X.*
* If you want to reinstall the OS and erase all data, select *Disk Utility.* Then select the hard drive in the left hand column. Select *erase* from the second tab, and then *erase* at the bottom right to confirm wipe. Then close Disk Utility and select *Reinstall Mac OS X.*
3. After the installation completes the machine will reboot.
Stay tuned for part two next week on how to create a bootable Lion or Mountain Lion USB flash drive.