How to recover a Mac from a Time Machine backup is a question that none of our readers want to ask, but occasionally have to. Sometimes the proverbial dirt just hits the fan.
All this assumes, of course, that you have backed up your Mac using Time Machine in the first place? You can't restore a Mac from a Time Machine backup if you don't have Time Machine set up in the first place. You have got Time Machine set up, haven't you? Just in case here's a quick primer.
Creating a backup using Time Machine
Time Machine was designed by Apple to work with a spare USB drive. Setting it up couldn't be easier. Attach a new external USB hard drive and click Use As Backup Disk in the dialog window that says "Do you want to use this as a Time Machine drive?"
The USB drive will be wiped so make sure it doesn't contain any files you want to keep. And although you can move files to and from the attached drive using Finder, we advise you to leave it alone and only use it as a Time Machine backup.
Configuring a Time Machine backup with a second hard drive is simple. The greatest effort it takes is purchasing the drive and plugging it in. But using an external drive won't protect you from a cataclysmic event —a massive power surge, fire or flood
Do you really need to reinstall Mac OS X?
First of all check if you really do need to re-install the whole drive. There are three things you should do before
1. Run Mac OS X Disk Utility from Recovery Mode
On a Mac running Mountain Lion, you can run Disk Utility by booting into OS X Recovery Mode.
Restart the Mac while holding down the Command and R keys.
You should boot into a screen headlined OS X Utilities.Click on Disk Utility and choose your Mac's built-in hard drive in the left column Click Verify Disk, and then wait while Disk Utility does its thing. If there are any problems click Repair Disk.
2. Run Mac OS X Safe Boot
Shut the Mac down, and start it up while holding down Shift. Safe Boot takes a while to start. If it does then you may be able to fix or repair Mac OS X. Try removing start-up items. Open System Preferences > Users & Groups choose you account, open Login Items and click the Remove '-' button to deselect options.
3. Run Mac OS X 'fsck'
This one's kind of geeky and fun. Shut the Mac off, and start it up again while holding Command and S. You're launching Single User Mode. You can release the keys when the intimidating black screen with messages in white text appears.
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