<File Missing> indicates that the file path stored in the external data source reference no longer points to the file that it should.
Instead of double clicking the line to edit the relationship. Double click the table occurrence box and check your external data source reference. You can correct the reference there by creating a new reference to your file or you can use Manage Data Sources to correct the error in the existing reference. I'd use Manage Data Sources if there is any chance that you have othere references to this same file in scripts, other layouts, value lists, etc as this will correct the problem for these references at the same time.
Just checked Manage External Data Sources (another innovationin FM 11 over 5.5). The desired file did not appear on the list. however, the file does exist in the folder (I keep all related files in one folder). I simply opened the file. Voila, now the relationship appears valid, and I can see the data, and all is well.
But it leaves an unanswered question: Why didn't FM open the file when it was prompted (when I opened that layout with the relationship linked to that file)? It opens all the other files as needed when relationships appear.
For now, it is fixed. Thanks!
With out the external data source, it had no way to find and open the file. This is one of the reasons why it is recommended that you convert a set of linked older files to FileMaker 11 format all in a group. That's supposed to make sure that each file reference is correctly converted and updated.
Thanks for the help. This seems to be fixed permanently.