The file that you are copying originally came from a Macintosh. Macintosh files consist of two parts, a data fork and a resource fork. You are trying to copy the file onto a DOS disk. On the DOS disk it is impossible to retain the resource fork. Should you continue with the copy, only the data fork will be placed on the DOS disk. In the event that you were to copy the file back to a Macintosh, the resource fork would no longer be available, and the application accessing the file might fail. For more information on resource forks, see your Macintosh documentation.