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Are you sure the map card is of a size that the Legend HCx can handle, and is proper for it, that is to say, the right map file on it for it to use?
If so, then what is the SD map type, and what error message do you get?
Try going to
Setup - Tracks - Setup - Data Card Setup
and tell me what options there are, there. The right option to use the data card may be there.
Or you may need to use MapSource software, to upload maps to this device.
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