diff command and saving results to a file

I have read the man page, and I think I am overlooking something, or am entering something wrong. I am trying to use the diff command, between a file on my pc, and one on my portable hard drive. Is there a way I can send the results to a txt file, or a file I could read in an editor? Or does this save the results automatically, and I just don't know where? Trying to compare directories, of files, between machines, before rebuilding. TIA Randal