Thanks guys that does the trick!
The only thing I'm finding now though is that the rectified raster now has a big black square around it. Is there anyway of removing this or making it transparent?
I have the same problem with black or green areas and rectified rasters. It seems to occur more when I have to rotate the raster being georeferenced. Yes, I can reset the band values to zero to make them transparent, but I'd like to permanently set them to nodata so I don't have to adjust the image each time I bring the raster into a map. Suggestions?