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If a Time Machine backup on your Mac is interrupted
If you accidentally disconnect your backup disk during a backup, or there’s a network problem, Time Machine may not be able to finish backing up. When the problem is corrected, Time Machine resumes.
If Time Machine is interrupted, when it resumes it may take a little more time than an uninterrupted backup.
To start a backup of your Mac at any time, click the Time Machine icon
in the menu bar, then choose Back Up Now. If the Time Machine icon isn’t in the menu bar, choose Apple menu 
> System Preferences, click Time Machine, then select “Show Time Machine in menu bar.”