Hello and merry christmas to the collective AULRO wisdom machine

 I got my Dad a Garmin Oregon 400C (waterways navigation charts) GPS for Christmas. I also bought the land topo maps on SD card for it. I have done a search here and google and can't find an answer and someone wants to go out and test it.
 The instructions are a little ambiguous, in that there is a dire warning that overwriting the internal memory maps will mean they are lost forever and Garmin won't replace them.
 The instructions don't say if the unit will read the maps straight from the SD card without overwriting the internal memory.
I guess most Garmins would be similar: Are you able to keep the internal memory maps whilst using SD-based maps?
Thanks for any responses
-Mark
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