There is a mystery here that I can't resolve: I had waterfall photos on the card is slot one on the D3x. I imported those to LR and put the card into my D3s and took some photos. When I extracted the card for import, only the waterfall photos where visible in LR import; however, it seems the images went to slot two. This is how I remember the scenario.
Now, there is no doubt that I am confused about this since what I learned here does not fit what I remember. In any case, I now have a clear understanding of how this works.