What is the recovery partition?
The OS X recovery partition is a tool that is built into all Macs running OS X 10.7 (Lion) or higher. The recovery partition is a collection of system recovery tools that enable users to diagnose and fix their Mac if it is unable to boot.
When to use the recovery partition:
If, when your turn on your Mac, it displays this flashing-question-mark-folder-of-death:
Then your Mac is telling you that it is unable to find a bootable device, meaning that it is unable to launch Mac OS X. This usually means one of two things: either that your Macintosh Hard Drive has failed or that your installation of Mac OS X has become corrupted. In either case, your first step should be to boot into the OS X recovery partition in order to diagnose the problem.
The recovery partition is accessible even if your computer is able to boot, and can be used at any time to re-install Max OS X in the case of a virus or operating system corruption.
How to boot into the OS X recovery partition:
In order to access the recovery partition, power off your Mac. Now locate both the Command and R keys on your keyboard, and start your Mac while holding them down. Continue holding Command-R until the Apple logo appears. Once your computer has finished booting into OS X recovery, you should see the following screen:
Note: If your computer instead boots to the flashing question mark folder screen or a Mac login screen, then you did not hold down Command-R early enough or long enough, and you should turn off your Mac and retry.
Using Disk Utility to check your hard drive:
If your computer was unable to boot (flashing question mark folder) then your first step is to determine whether or not your hard drive is physically damaged. Hard drives are the computer components that break most frequently as they contain a disk that spins thousands of times per minute, and are relatively prone to breakage.
To check your hard drive, first click Disk Utility in the Mac OS X Utilities window. The Disk Utility window will pop up:
On the left menu, Disk Utility will display all storage devices present on your computer (most people will only see one device, their primary hard drive). Select the drive that Mac OS X is installed on, (usually called Macintosh HD) and then click Verify Disk. When it is finished, Disk Utility will inform you of any problem that may exist on the drive. If it does not find any, then it is likely that your Operating System is corrupt.
If Verify Disk does find problems with your drive, then click Repair Disk. What Repair Disk does is to scan the hard drive and fix any inconsistencies it identifies in the way that files are stored on the drive. Repair Disk will not help if there are areas of the hard drive that are physically damaged.
Note: Both Verify Disk and Repair Disk will list the errors they detect/fix. For more information about what each error means, check out Apple’s Disk Utility Documentation.
Using Recovery Partition to run a Time Machine system recovery:
If you have configured and backed up your computer using Time Machine, then it is possible to recover your computer to the state it was in during one of your Time Machine backups.
To do this, simply plug in the external drive that your Time Machine backups are saved to, and then select the Restore from Time Machine Backup option in the Mac OS X Utilities windows. This will bring up the Time Machine recovery user interface that will allows you to inspect each Time Machine backup present on your drive, and to select the one you want to restore from.
Reinstall Mac OS X using the Recovery Partition:
If your Mac won’t boot, and you have verified that your hard drive is working correctly using Disk Utility, then your installation of Mac OS X is corrupt, and requires reinstallation. Note: This process will delete all your data and installed programs, and will install a fresh version of Mac OS X.
Mac OS X reinstalls itself by downloading a copy of the version of Mac OS X that came with your Mac from the internet and then installing it on your computer (you may need to log in with your Apple ID in order to begin downloading).
To begin, connect your computer to the internet using either Ethernet or wireless and then click Reinstall Mac OS X from the Mac OS X Utilities window. Note: For those using wireless: only wireless networks with certain encryption types will work with OS X recovery. For UMass Students: The type of wireless encryption we use with both the UMass and UMass-Secure1x wireless networks will NOT work with OS X recovery. In order to re-install OS X you must use a properly configured home or public network. For more information about which wireless network types are supported, visit Apple’s OS X Recovery Documentation.
Note: Sometimes Mac OS X install will fail. If this happens, one common issue is that you need to reset your NVRAM. To do this, shut down your computer and then start it while holding down the Command, Option, P and R keys simultaneously. Hold these keys down until the computer restarts and you hear the start up sound for the second time.
For more information visit Apple’s OS X Recovery Documentation.