He is correct though, just because the GP has not heard of it doesn't mean it isn't commonly used. I'm also a German native speaker and I agree the term Zettelkasten is more common than Karteikasten. I encountered Zettelkasten when I was in high-school a long time ago. That said the way I've encountered it was always more as a study tool (like flashcards, or the vokabelkarten/box the GP mentioned ) not so much for note taking, but then that's what you do in high-school.