SS1122, I also have a 276C, as far as I know you can't access the Data card memory directly like you can with a SD card, all you can do is to transfer Maps to the unit (or your card reader) through Mapsource.

When you connect the data card either via your card reader, or connecting the whole unit via USB cable to the PC, it doesnt come up as a seperate drive(in My Computer) as, say, a SD or other memory card would.

You should have no problems transferring any compatible chart or map to the unit through mapsource though. I did have a problem with my Data card reader, but no problems when the unit(+data card) connected directly to the PC via USB, Mapsource recognised it, so I'm not sure why you cant send maps to the unit via mapsource. Have you tried using a differant version of mapsource?

You will need the lastest firmware update on your 276C (276 firmware version 5.40) to work properly with the 512mb cards, but any version should work if your card is only 64mb. It may be worth updating the firmware anyway, just make sure the battery is fully charged on both the 276 and your laptop as a power out during update will zap the unit.

The 276C is a fairly dated(but still excellent) unit, so check compatibility with newer mapping/charts.