1) put the main-db and the lookup-db in the same directory (that is much eassier)
2) open the manage-external-data-sources dialog in your main-db
3) Change the filereference there to the correct path
4) Remove the references to (the same file) that you don't need
From your main Db.
File-->Manage External Data Sources
Look for the associated file link....it is probably missing or pointing to an invalid location.
Outstanding advice. Works perfectly. Thanks very much!