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Backup your entire computer (Part One)
If you have never used this program before, be aware of its power. There is no 'undo'. Beyond Compare is a very sharp knife. Don't just close your eyes and click "Copy Entire Drive".
Mirror will be overwrite everything at the destination. If you have unique files there, they will be deleted. Update is safer. It will not delete anything outright. The worst it will do is overwrite newer files over older files with the same name.
You are better off doing your backing up in parts. Do the smallest and least important part first. Do all your photos. Do all your music. Do all your archived emails. Do your financials.
Before you start, set the log to be archived somewhere. Depending on how the operation turns out you will have varying reasons to go back through the log. There is a good chance you won't need it at all.
When you are all done with your copying session, the backup will have a slightly different size than the source. You will need this log to inspect after you finish backing up.