I have the Garmin GPSMap 60CS (old, 10 years or more).
On Garmins website for firmware, its says the current one is 4.20 from 2007, and one of the updates says this:
Changes made from version 3.60 to 3.70
added advanced NMEA support settings to support certain marine autoplot units
this I why I'm thinking its firmware. Ishmael may be right about the firmware not destroying the charts (which are downloaded, not removable data card)