Make a Time Machine Backup
Do not even think about installing the macOS High Sierra beta without making a current backup of your computer on macOS High Sierra or OS X El Capitan.
This is a beta and you may find out that some apps do not work at all or some functions are missing. If you have a Time Machine Backup you can downgrade from the macOS High Sierra beta to macOS Sierra quickly and without losing data. Users who don't have a backup are in for a bad time.
You can use this guide to make a Time Machine Backup of your Mac to a USB hard drive. If this is the first time you are doing so, it will take a while to complete but it is worth it.
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