It sounds like the file was corrupt at its source. You can try this: Open Disk Copy, go to prefs & tick 'Ignore invalid checksums'.
Also take a look at these 2 threads: http://www.gnutellaforums.com/showth...threadid=24703

http://www.gnutellaforums.com/showth...threadid=29131 Altrnatively, visit the Mac site & search the Support section for this type of error.