@dj_paige, thanks for the suggestion. However, the page you referred to talks about moving pictures on the same computer, and about reorganizing folders on a computer. That is not what I am trying to do. I have the same pictures on two computers and am just trying to transfer the metadata from one computer to another, and LR won't recognize the pictures as being the same when I try to import them.
The instructions describe how to point Lightroom to the new location of the photos. This is not an import issue, this is a "Find Missing Folder" issue on computer 2.
Partial success, but I still don't understand why it worked (and why it didn't work!)
Prior to posting this thread, I had tried what is suggested in the link re finding the missing folder, but I repeated it so that I could report the problem I was having with that approach accurately. When I point the missing folder (on the c: drive) to the original folder on the D: drive, LR says that the folder is already in LR, and asks if I wish to merge them. I tell it to do so. Now it gives me a warning message that files of the same name exist in both locations, and that the duplicate files and folders have been left in the original locations. I ask it to create a folder of the duplicate files, and it consists of all the files from C:\, with the metadata. WHen I look at the folder in D:\, none of the metadata has been transferred. So now it seems to recognize the pictures as being the same, but would not do so when I imported the catalog.
So I tried an alternative--fortunately, for once, I had created a backup! :-) I removed the subfolder on the D: drive (just remove, not delete) with these pictures from the original catalog on the D: drive, and imported the new catalog. It again said that it had found new pictures, and I told it to add them without moving. Once again it added them to the C: drive. I pointed the folder to the D: drive, and this time it accepted it, but here's the interesting part. What it did was to reload the subfolder on the D:\ drive (the one that I had previously removed) AND added the metadata to the pictures in it. With regard to the folder on the C: drive, that still shows all the pictures as missing. When I tried to remove the folder on C:, LR froze, so I closed and reopened it, when the folder had been removed. The folder on D: seems to have the metadata still in it.
I'll try this later with other subfolders that I have to transfer, but I still have my original question, which is why does LR not recognize that the pictures are duplicates when I first import the catalog, but then changes its mind and does recognize them when it goes to merge them. Besides that, this is a really tedious way of doing something that (to my mind) is analogous to exporting a catalog from a work machine so that you can add metadata to it on a home machine, exporting the catalog on the home machine, and then reimporting that new catalog on the work machine (as described by Julieanne Kost in her video LR - How to: Move & Archive Images, and Export a Catalogue | Adobe Evangelists - Julieanne Kost | Adobe TV starting at 6:20)
Sorry for all that information, but I have tried to provide you with what happened every step of the way.