[dnanian]

You can't really do that without requiring substantial extra disk space needs -- imagine, for example, copying a 2+GB Entourage image or huge modified movie file to the backup -- you'd need a lot of potential disk space for every file copy, rather than just the occasional exception.

We don't erase the backup first -- we copy over it. (The OS is going to do the erase in its own file copy function, or perhaps just replace the data portion and leave the file in place.) But, the end result is the same.

But, really, this is hardly the main case to worry about. There are lot of similar issues that could occur, including a failure of both the backup media and source drive, and rotating a backup -- or, as I suggest in the guide, three -- is the best way to reduce the probability of this kind of thing hurting you to near -- but not at -- 0.