Thanks for your response. I am however not sure I do understand everything correctly:
Sometimes you have to wait a while. After relinking to the highest-level folder you can, give LR some time and see if everything gets updated - it should.
What should I link to the highest-level folder? Clicking on a red folder LR asks for a link to the missing folder and doesn’t do anything when pointing to a folder higher up in the hierarchy. So is there any function like “all of my images are within this folder?”
For this purpose, rather than having all your separate folders at the disk root, it helps to actually have them inside one folder. That way you can just relink the top-level folder and everything underneath comes in too (eventually).
I have done so but it does not help. Even waiting for hours LR does not really get the simply the top folder has changed.
Also, I've noticed that LR on Mac is much better at keeping track of files than it is on Windows.
Well, in reverse this means even when doing everything right, LR might fail? If I remember correctly, Jeff said so in the tutorial, too.