FileMaker Pro 188.8.131.520
The FMP app cannot save changes to a DB stored on a mac Disk Image (".dmg" file).
The dmg file is writeable. Past versions of FMP had no problem with this.
Work-around: copy the DB to local disk, update the copy, then copy it back to the dmg file (replacing the original).
Not sure if the problem also occurs with mac Spare Disk Images (".sparseimage" file).