I have both my photos and my LR catalog stored on an external drive. With this drive plugged into my MacBook, I did some work on my photos in LR. This included renaming the files using the Library > Rename Photos... option within LR (I did not use Finder to rename my photos).
When I later attached my external drive to my iMac (different computer), and opened up my same LR catalog, I noticed that LR still thinks all of my photos still have their original filename. I have 57 photos in my catalog and it seems that I have to go through one by one and relocate them to the external drive where they are now stored with their new filename.
Why did LR not have the files associated with their new filename when I opened my catalog on a different computer?
I suspect you aren't opening the same LR catalog that is stored on the external drive. Have you checked your internal Macintosh HD for a LR catalog file? Search for .lrcat on the internal drive.