i had the problem... but annoyingly enough, i can't remember how i fixed it! lol
i think it was something to do with permissions... either on the file or directory itself, or on the samba share.
just as a test, can you try and chmod the file to 777 and see if it'll copy then?? create a new useless file to test it with if you'd rather not chmod important stuff to 777. Also... what's it chmod'd to at the moment? 644?