Occassionally I just want to add records that are in one source (excel sheet) but not yet in a FileMaker database.

You can do that using 'update matching records in found set' and the 'add remaining data as new records'. However, you still have to choose a second field to actually update or it will complain "No fields were selected for import.".

This seems like an odd restriction for FileMaker Inc. to build in. Sometimes I want to import only the ID fields that don't currently exist. However, I'm forced to import/update an additional field to get by the warning.

This isn't the end of the world of course but just an odd thing for them to go out of their way to prevent us from doing.

If you want to import only the ID fields that don't currently exist why are you using the 'update matching records in found set' option? You don't want to update matching records, you want to add new records.

The ID field should have the following options selected on the Validation tab: Always, Unique. When you import ID's use the Add Records radio button. Any duplicate ID's will not be imported because they dont pass the validation.

Try it on a test file.

Excellent point, thanks for pointing that out.