I could have a folder of stuff. But I don't have to. I could make the folder of stuff into a .dmg file then when I click on it the .dmg file mounts on my desktop. In the mounted folder is my "stuff". Why? OK I am able to encrypt the .dmg but is this the only reason for the existence of .dmg or does it have other qualities. What does it mean exactly that a .dmg file will "mount" on the destop. What is the difference or relationship between the .dmg file and the image that 'mounts' on the deskop that looks like a HD icon.