I asked the same question a few days ago, and I discovered Encrypted DMGs. But is it possible to make an automatically adjsuting DMG, this means: when I add a file to the DMG it adjusts it's size to that file. Example: I drop a 50MB PSD on this DMG, but this DMG is only 20MB. Is it possible that this DMG becomes 50MB to accomodate the 50MB file, automatically ?