When you use Find Missing Folders, the photos are now stored in a different place in the Folder hierarchy shown in the Folder panel. Why? Because you just told Lightroom so.
If the folders were on Drive A and under parent Folder XYZ, and you told Lightoom that the folders are now on Drive B under parent Folder XYZ, then you must look in Lightroom under Drive B, not Drive A.
Thank you very much for the quick answer, but I don't really follow what you mean.
The files were not moved from one drive to the other... I just unplugged the external HD from the laptop and plugged it into the desktop computer. I simply moved the catalog file (without the preview files). When I open the catalog files, I get empty photo frames. But once I updated the root folder location in Folder panel (to the same drive that is now on the new computer)m Lightroom asked me something along the lines of "it seems you already have this folder in the catalog, do you want to merge the two folders)". I say yes, and the folders in the folder panel are no longer missing ("?"), but Lightroom finds no photos - even the count of photos (in the folder panel beside the folder icons themselves) says 0, although I pointed lightroom to the the right external drive.
I have also found that that the catalog file I copied from the laptop seems to differ somewhat (folder hierarchy in explorer) from what is actually on the external HD.
Sorry, my bad. I have tried using another catalog file from my backup, and it would seem that it was only a corrupt catalog file. It works fine now.