A moment ago I tried to do some "develop" work on a file in a particular collection. When I did that, it showed the image (a little funny looking" with the message "the file could not be found. I am using a Mac, newest version of LRCC. LR does not do that in other collections. This collection I want to work on is not a smart collection. What's up?
Then you moved the image from one folder or drive to another folder or drive or the external drive the images was on when it was imported is not connected to your Mac or it is not being seen by your Mac if it is actually connected.
That error simply mean the images is not exactly where it was when it was imported into the LR catalog.
Get the File name from the Library module, Metadata area, and search your computer for that file name using Finder.