> Does HTTrack checks the content of the files, or 
only 
> the name, and the size ?
The update process uses either an internal ID (Etag, 
see RFC2616), or the internal date stored in the 
cache, asking the server whether to update or not.

If the 'update hack' option is used (which is the 
default, maybe a bad idea..), the system will NOT 
retransfer data which has the same size than the old 
copy (assumed identical)

Ah, a last thing: the complete structure is always 
rebuilt, even if no data is being (re)transfered, 
because the engine don't know if you modified some 
options (like filters or structure type) which may 
have modified the local structure..