Use the Time Machine feature on Macs running OS X Yosemite to restore data from a backup. With Time Machine, you can restore your entire system to a previous state or restore individual files while keeping the rest of your current system intact.
Restoring Your Entire System
Make sure the external hard drive or disk you use with Time Machine is connected to the Mac and turned on. If you use a networked drive, it must be on the same network as your computer. Verify your Mac can see the external drive by opening the Finder and looking for the drive on the left of the window. When you're ready to restore the backup, click the "Apple" menu in the top corner of your screen and select "Restart." When you hear the chime, press and hold down "Command-R" on your keyboard until you see the Apple logo. Select "Restore from a Time Machine Backup" and click "Continue." Select the backup disk, enter your username and password, if prompted, and then click "Connect." Select the backup you want to restore and initiate the restoration.
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Restoring Individual Items Using Time Machine Only
Click the "Time Machine" icon on the menu bar along the top of your screen and select "Enter Time Machine." The Time Machine icon resembles a circular arrow with clock hands inside of it. If you don't see this icon on the menu bar, click the "Apple" menu, select "System Preferences," click "Time Machine" and then "Show Time Machine in Menu Bar." Open Time Machine and then browse through the timeline to locate the backup and choose the items you want to restore. Press the "Restore" button to return them to their original location on your Mac. If a document was originally saved on the desktop, for example, Time Machine returns it to that location.
Restoring Items Using Time Machine and Spotlight
If you are unsure which Time Machine backup contains the items you want to restore, use the Spotlight feature in the Finder to locate them. Click the "Finder" icon on the Dock to launch the Finder application and then click the drive you use for your Time Machine backups. Enter the search terms in the search field at the top of the window to locate the correct backup file. Make a note of this location, click the "Time Machine" icon on the menu bar and select "Enter Time Machine." Locate the backup file you identified, select the items you want to restore and click the "Restore" button.
Troubleshooting Time Machine
If Time Machine has trouble locating your backup disk, confirm the drive is connected to your Mac and turned on. If the drive is connected to a different computer on the same network, make sure the other computer hasn't fallen asleep or been turned off.
If the drive is present in the Finder but Time Machine still has trouble locating it, verify Time Machine is configured to use the correct drive. Click the "Time Machine" icon on the menu bar, select "Open Time Machine Preferences" and verify that the correct backup drive is selected.
If Time Machine is slow, disconnect from the Internet and disable your virus-protection software temporarily.
If the process of making backups slows down your Mac, schedule backups to take place overnight when the computer is not being used.