yes lol,update its software, it's too old now is GPSMAP 420/420S software version 5.60 :
in even old sw 4.00 in changelog is written: Added support for SDHC cards, which have capacities greater than 2GB.
Edit: additionally some new BC are not supported by old firmwares. Read all the changelogs, many features added and many bugs corrected .

Code:
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