Gpsmap 620 is somehow a special unit.
It has two modes, Automotive and Marine, and when you switch between these two modes the unit reboots and starts again as a Nuvi or as a Marine chartplotter.
When it is in Marine mode it follows Garmin chartplotter rules, so only 1 Bluechart map per sdCard.

Momotaro check the version your firmware and if it 3.70 or 3.60 you are OK, if it's a previous version use Webupdater or Garmin Express to update your unit.
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