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Long ago, in the days of DOS and OS/2, deleting a file meant that it was gone for good. How file systems worked was a mystery to just about everybody, and so we were told to back up our data often lest a mistake or drive crash wipe out something important, leaving us up a certain body of water sans propulsion. Years passed, as they are wont to do, and someone discovered that data didn't really evaporate when it was deleted, it was just renamed in such a …