I think what you want will work if the External Data Source Reference that you can open from Manage | External Data Source in FM Pro uses a relative path to reference the file from the related table. That reference should look like:
That should, however, be the type of entry that is made automatically if the two files are in the same folder on your computer at the time that you first add an external data source reference to the 2nd file.
Great! Thank you.
Not sure what I was doing wrong before as I was selecting a file in the same directory,
but did it again and it works just fine..
File Menu | Manage…| Database… | Relations Tab | Double click on existing or New Table button bottom left
Specify Table ( dialog ) | Data Source Menu | Add File Maker Data Source, or Manage Data Sources.