A slightly different spin on tedd's approach of using a second hard 
drive is to use Speed Doubler 8's copy. It automatically intercepts 
the drag and drop Finder copy (OS 8/9) and provides a "Smart Replace" 
option. "Smart Replace" only replaces the files on the target HD that 
haven't changed. This way my backups (even though I drag and drop all 
folders) take only a few minutes after the initial backup. Maybe 
someone can write a smart replace copy utility. Just a thought.

Brian