A manual backup (separate from Time Machine) of any interim documents or data created between the time of the Time Machine backup and now*.Time Machine backup made prior to updating to macOS Sierra (either with Mac OS X El Capitan, Mavericks, Yosemite, or otherwise).Requirements to downgrade from macOS Sierra to prior Mac OS X The ability to prevent data loss, or roll back problematic software updates or OS upgrades is invaluable. This obviously requires a functioning Time Machine backup to work, one of of many reasons why setting up Time Machine on a sufficiently large external drive is one of the most important maintenance routines you can have on a Mac.
