Magellan RoadMate 300 Reference Manual page 42

Maps
The Map Window and process for drawing regions and uploading them to
an SD Card vs. an internal memory are identical. The upload information
reflects the SD Card only. The Magellan RoadMate Manager software
limits the size of map files to 240 MB. This is to reduce the amount of
time it takes to process and upload the data. See page 18 for
recommendations on Map File size.
Do not disconnect power at any time during the upload process or you risk
damaging the data. When you have finished uploading, you can turn the
unit off and disconnect the USB cable and power supply from the
Magellan RoadMate.
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