If you have more than one user account, you must be logged in as an administrator to carry out these instructions.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Triple-click the line below to select it, then drag or copy into the Terminal window — do not type — and press return:
sudo defaults delete /Library/Preferences/com.apple.TimeMachine.plist SkipSystemFiles
You'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up. If you don’t have a login password, you’ll need to set one before you can run the command.
Reboot and run a backup. Check that the previously excluded files are now included. The backup may take much longer than usual.