I recently copied my LR Classic library and my catalog from my internal SSD drive to an external SSD drive to free up space on my internal drive. I used a back-up from the day before I started this operation. I then re-synced my top-level folder so that LR would know where the photos were and also updated my catalog so that LR knew it was on the external drive. It appears that all my photos are on the external drive and LR knows that. My problem has occurred with my collections, they all refer back to the original folders and photos on the internal drive. And those that were synced with Adobe cloud are empty. Looking at my account on Adobe, these photos are missing too. Also my photos in Adobe Portfolio and Adobe Express (used to be Spark). I also use the Book module and all my saved books are empty.
Any thoughts on how to reestablish my collections short of recreating them one by one?
You re-synced your top level folder and that was in effect a fresh import of all these photos.
Instead (and I suggest you go back to your same Catalog backup) re-browse this top level folder from the old location, to the new. This transfers everything and the images which were showing as offline (source files not found) will all reconnect and resume working as they did before - only, now referring to the different drive.