Sunday night: I edit the start of chapter 3, which I haven’t worked on since early December. I place the entire novel folder onto my brand-new USB key (thanks, Rhys) which has nothing else on it and thus there is no danger of overwriting anything.
Monday afternoon: in the library, I open chapter 3 and find that in fact I did not save my edit of chapter 3. I have only the early December version. I try in vain to blame the computer for my own stupidity, and edit the start of chapter 3 again.
Monday night: I get home and find that I did, in fact, save my edited chapter 3, but transferred the folder before saving the edits. So the computer has the original edited version (and the original non-edited version), and the USB key has a different edited version.
Well, at least I didn’t lose any work after all.
Punchline: the second (Monday afternoon) version was the superior one. Nevertheless, I don’t intend to make this sort of thing a habit.