Re-downloading a torrent increases your downloaded amount and decreases your ratio.
But chances are, if you can copy the files to a "properly working" drive, you can "repair" the files by simply "assigning the target folder of the torrent to the new location for corrupted files" and reuse the portions of the file that were not affected from the corruption. In this case, you wouldn't have to download the entire file.
However, while copying if you get a CRC error, that would mean you can not copy the files to another drive. In that case, you might have to re-download the entire file or copy the files with a command prompt utility called "xcopy" with "/C" parameter (Continues copying even if errors occur.) and recover as much data as possible.