I've just upgraded to Lightroom 6. I have my photos on external hard drives. When I search and find photos using the exclamation mark on the thumbnail, and click open, Lightroom 6 says "each file can only be associated with one photo" . I tried importing the photo again, still get the same message. I tried searching for the issue, but didn't find anything.
Any help would be appreciated.
This means the photo is already in Lightroom, and you can't import it again, and you can't fix exclamation point icons by pointing to that file. You need to find the photo inside of Lightroom, and make use of that.