Open Manage | External Datasources
Update the entries in here to text that works with both locations and this error should disappear for you. You may want to look this section up in FileMaker help to better understand your options.
See? I would never had found that. I don't think of the other databases as "External", though now it does sort of make sense. They're "external" to the one being invoked.
Don't think of them as databases. Think of them as what they are: Files. A database can consist of many files or just one file, depending on how the developer structures the system.