I was poking around in Leopard's Disk Utility and it lists a "sparse bundle disk image" as an type of image to make. The help mentions how to create it, but not how it might differ from a sparse disk image. Apple's site has no info on it either. Closest info I found was this definition: http://eng.proz.com/kudoz/2219600 Could this be a new image format used to enable the Time Machine functionality? It seems it may grow and shrink on the fly (as opposed to the old sparse images which only grew, but had to be manually shrunk).