The folders don't disappear. They move to the new disk.
If, for example, on the Mac, your catalog showed folders under Disk A, and then you perform this process to indicate to Lightroom that the folders are now on PC Disk Z, then in Lightroom you can no longer find the folders under Disk A, but they will be found under Disk Z.
Thanks, but how do I now find the 2011, 2013 and 2014 folders in the same Lightroom catalog?
You find them the same way
If you watch the video, you see that the other folders that was located on the same disc disappears, and I have no way of finding them again.
and I have no way of finding them again.
In your catalog, go to the Catalog Panel on the left, select "All Photographs" and then search for the 2011 or 2013 or 2015 photos by using the filter bar or a smart collection, or just scrolling down until you come to one of the 2011, 2013 or 2014 photos.
Thus, you have a way of finding them again.
Once you find one of these photos, right-click on it and select Go To Folder in Library. This will locate the folder containing this photo in the Folder Panel.