I'm new to the program and I'm curious about the catalog back-up that it frequently asks me to perform. When I back-up my catalog to my external drive, it appears as a single icon for a file of about 97Mb. My actual catalog has something near 10,000 photos and is about 30Gb in size. Is the program compressing my files to keep them backed up somewhere, or is it only saving thumbnails of each image, or...? Help! Thanks!!