copying files (overwriting)

[René]

hi gnome users
i want to copy files to an other harddisk, but only newer versions of  
files or complete new files should be written to it. Nautilus now only  
asks me, if i want to overwrite a certain file which does already exist. I  
want to understand with which 2 versions of a file i am dealing at such a  
moment. how to achieve that? response on that would be kind.
regards, René