Assuming you are refering to a Garmin device, you can use the internal memory to copy two maps, one named gmapprom & the other gmapsupp. You can use your SD to copy two more files one named gmapsupp & the other gmapsup2 not gmapsupp2 Garmin won't recognize it. If you want more info about mapset naming you can find an interesting mapset naming guide here: (link removed)
Spoiler:
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