> In the morning, I resumed (?) the download, but HTTrack was 
> finished very quickly, and then proceeded to 'purge' 
> everything I had downloaded earlier!

I experienced something similar recently.. I was working on a project but
cancelled before it had finished and HTTrack proceeded to delete a lot of the
files (but did delete not folders).

At the time I didn't care that it had done this and thought it was something I
did wrong...  I wish now I paid more attention to what I did.