Ok I just noticed this and it might have caused me some problems that I have yet to find out.
So, what happens is, I relocated all hammer files to this other computer and If I create a new prefab in one of the old prefab categories, they dont appear if I close and restart hammer!? Only prefabs created in a new category will appear in the next session? And, the new prefab categories don't appear in the prefabs folder? How is all this prefab stuff organized?