Anybody know why some areas don't appear on my GPS but DO appear on my PC?
That usually indicates the area was not selected in MapSource/etc. and therefore not transfered to the GPSr/data card.
Yes, if you see a cross-hatched area on the GPS, it means that no map is available. You need to be certain you're selecting all the tiles that you need in Mapsource and sending them all to the GPS.