This will reset the connection to your backup destination, and replace the preferences file containing your settings for Time Machine:   the destination, options, and any exclusions.  If it's corrupted, Time Machine may not work properly, if at all.

It will not delete anything in your backups, or cause Time Machine to start a new set of backups.

1. 1.On the Time Machine Preferences window, turn Time Machine OFF and be sure a backup is not running (turning Time Machine off won't cancel a backup that's already running).  If in doubt, see item #D6.  
   

2. 2.If you're backing-up some things, but not everything, on any external HDs, connect them temporarily (otherwise those exclusions won't appear in the next step, so you might forget them, and won't be able to re-enter them in step 6). 
   

3. 3.Click the Options button on the preferences window.  Note everything in the exclusions box, and the settings of the other options (perhaps via a screen print), then click Cancel to return to the main window.  Then quit System Preferences.

4. 4.If you're backing-up to an external HD, eject, disconnect, and power it off for a few moments, then reconnect it. 
   

5. 5.Delete the file /Library/Preferences/com.apple.TimeMachine.plist  (in your top-level Library folder, not the Library folder inside your home folder, where most user preferences are).
   

6. (A Spotlight search won't find it, and neither will a Finder search, unless you include System Files).
   

7. If you have trouble finding it, from a Finder menubar, select Go > Go to Folder, copy /Library/Preferences to the prompt and click Go.  Scroll down until you find com.apple.TimeMachine.plist
   

8. Or you can navigate to it, by opening your Startup Drive, usually named "Macintosh HD," via the Finder.    If you have a problem finding the file, see item #A7.)
   

6. 6.Go back into Time Machine Preferences.  If your backup drive is still selected, the contents of the old file are still there.  Restart your Mac, and delete the file again.
   

7. Re-select your backup drive, and re-enter any exclusions and other options you noted in step 3.   If you're backing-up any external HDs, they'll be excluded automatically, so remove those exclusions.  If you're not sure how to do that, see Time Machine FAQ #10.
   

8. Note:  If you're backing-up to an external HD, it's shown automatically, in gray, and you can't remove it.  Any drives or partitions that aren't formatted for a Mac can't be backed-up by Time Machine, so they'll also be shown in gray, and can't be removed. 
   

9. The oldest and latest backup dates won't appear on this window until the next backup is done.  "None" does not mean the backups are lost, just that the dates haven't been re-loaded.
   

7. 7.A 120-second countdown to a backup should appear.  If you close the preferences window, it should begin immediately.

6/9/13

Copyright © 2013 James Pond.  All rights reserved.