When using the map Topo Deutschland in MapSource or QLandkarte there are grid lines marking rectangular map slices which can be selected for transfer to the Garmin handheld.
But when using OSM maps generated by user Computerteddy there are additional lines. When selecting an area of the map only parts of the map are highlighted for transfer to the handheld. It seems like vacancies in the transferable map. But on the handheld the map is complete.