file already exists error when copying files to external hard drive

I am trying to back up files to a my book 500G ext hard drive. I have been copying / pasting entire folders (uncompressed for now). I have been getting "file already exists" error and the copy stops at that point. No option to replace file or continue without replacing.... all copy just stops at that point. Short of selecting individual files for copy, how can I bypass this error? This error occurred on the first copy attempt, even though no files were copied yet, and of cours when I reattempted the copy.