LR sees all photos on one external drive, but only 80% of same photos on the other external drive

My photos are stored on an older external drive and a fresh new external drive (in preparation for replacing the older drive). Finder indicates that "Images for LR" on both drives are the same size, and spot check seems to indicate all photos are indeed on both drives. The catalog is on my desktop. When I point the catalog to the older external drive , it finds all 47,000 photos. When I point it to the newer drive, it only finds 33,000.  How do I correct this problem?