It all sounds very very strange.
To say the least....it gets even stranger...
Nevertheless, I have finally managed to recover everything... I think. Though it was much less straightfoward than I expected. Thanks to those that offered suggestions and above all sympathy.
I thought I would summarize what I found in case anybody has thoughts/suggestions. It is still not clear to me if my problems were self-inflicted or not.
An important point before I begin. The catalog, catalog backup and images were all installed on the G drive of my old computer. When I copied these files from my old computer to the new, I put them on a different path on the C drive.
I double checked my catalog. The apparent reason that the A folders were ok and the B folders were not is because the catalog file was dated two months before the old computer crashed. In fact it is dated from the day I imported the B folders for the first time. Now this is really curious since I was working on photos in the B folder in this catalog right up to the day the computer died. So the catalog should have been more recent. How could the catalog be out of date but the XMP files up to date? I searched the hard drives I had removed from the old computer (they were not damaged) just in case I had in fact unknowingly been using a more recent catalog in a different location. I found nothing except that backups. The other thing I do not understand is how LR found the A and B folders on the new computer in the first place (there were no question marks)
since I had changed the path and did not reassign it when I opened the catalog on the new computer.
I also double checked my catalog backup which was dated the day the computer died. As indicated previously, all the B folders were ok and listed under the C drive. How did LR find the B folders in the first place since I did not reassing the path (there were no question marks). Moreover contrary to what I said initially the A folders are there as well. I had not noticed them previously since they were listed under the now non-existant G drive (in this case there were question marks). I reassigned the path for the A folders and everything is now back to normal.So how did LR find the path for the A folders but not the B folder?
In the end the backup worked, though there are details I do not understand.