I use a single catalog for all my photos.
To take advantage of Smart Previews, and do off-line editing, I copied the catalog files to a portable SSD, and did some editing while traveling.
You do the opposite thing. You copy the catalogue - just the lrcat file - back onto your main computer.
For example, I use a single catalogue held on an internal drive. Before smart previews I had to use File > Export as Catalogue to take on the road a mini catalogue of some pictures, and then use File > Import from Another Catalogue to bring the work back into the main catalogue. That's probably the workflow that you're imagining you should follow. But since smart previews before I go on a trip I copy it (ie the lrcat file) and its smart previews onto a portable drive. While I'm away I'll work on my laptop and use this portable catalogue, adding new photos and maybe modifying older ones thanks to the smart previews. At the end of the trip I copy the lrcat file back to my internal drive, copy the new photos to the internal drive, and I'm done.
If you copied the whole catalog from your main computer to the SSD, and did no edits on the main computer, just copy the catalog off the SSD back onto the main computer.
Alternatively, you could just use the SSD drive as your catalog drive and move the external drive from computer to computer as you desire.
Thanks for the quick responses. I haven't done any edits on the main computer, but I have added new photos.
When I copy the lrcat file from the SSD back to the primary machine, I expect LR will "forget" all those new photos, even though the images themselves will still be stored on disk.
Is re-importing the images (without copying) the correct way to bring the catalog back to its previous state?
Read how I do it. I take "the single catalog for all my photos" onto the portable drive. When I move it back to the primary machine, it does not forget the new photos but might not know where they are - eg if they are on the laptop drive. But they just show as missing. I then correct that by updating the folder location. No re-importing necessary.
I apologize for being ambiguous in my earlier post. The new photos were not added to the catalog on the SSD; they were added to the catalog on the primary machine. The catalog on the SSD has never seen these new images.
Since you have made edits to both catalogs, your best bet is to export the edits from the SSD and import them back to the main catalog:
Using the SSD catalog, select your edited images to transfer.
From the File menu, "export as catalog".
Copy this small catalog to your main computer.
Using the main catalog, in the File menu, "import from catalog", choose the small catalog you just copied.
You may need to "relocate" any "missing" folders then "find missing photo".
If you do these transfers on a regular basis, you would probably be better off just putting the catalog on the external drive and move the drive from computer to computer.
Thanks so much for the detailed instructions. Accomplished exactly what I wanted, and now I know a bit more about catalogs.
As an aside, my primary catalog is already on an external SSD, but that drive is not as portable (form factor, etc.) as the one I use for traveling.