Hi Severin, I don't know why disappearing disk space is a recurrant problem for you, but perhaps it is due to large cache directories (e.g., consider that apps like Adobe Bridge can just keep growing and growing, and some Adobe apps used to have a bug where they didn't release their temp files). A tool like "Disk Inventory X" will show you a graphical breakdown of your disk space (folder-by-folder).
I'm not sure if "Disk Inventory X" allows you to save or print its results for future comparison, but at the very worst you could take screenshots of its output. Assuming that you don't have a corrupt disk, I would have expected that the large difference in space between your original and mirrors drives is due to file-exclusion options in Super Duper (though I have never used it, so I don't really know). Disk Inventory X can show you "physical" as well as "logical" file size, so that would indicate the extent of the difference if any.
