I used Superduper to clone one external HD to another. After completion, I clicked "Get Info" for the two volumes. The source says Used: 989.77 GB on disk (989,768,949,760 bytes) and the destination says Used: 989.98 GB on disk (989,975,355,392 bytes). The target HD was erased before the procedure. I compared the Get Info windows for each subfolder on each HD and they exactly matched. What is the extra 206,422,016 bytes I am seeing? Some kind of file system overhead?