I have never tried to do this, not sure what SD card to buy. Will someone help guide me in the right direction please.
Since no one has answered your question, I will.
Units with Auto Chart Live have up to 8 hours of internal memory for creating HD contour mapping.
After that 8 hours, you have the choice of deleting the data or copying it to a Zero Line Card, then you get 8 more hours of Auto Chart Live.
You can leave the Zero Line Card in the unit to record the data directly to it, and have enough memory for the rest of your life. The card has millions of zero lines (shorelines). Auto Chart Live data is overlayed on whatever mapping background you have, Lakemaster, Navionics, or built-in mapping. When you play it back it is still an overlay.
You can also do RECORD which will record sonar logs to a regular memory card up to a 32 GB capacity. You can process that data via the computer software Auto Chart or Auto Chart Pro. When the data is processed, you then copy it to the included Zero Line Card and it will be read by the unit as Lakemaster==same settings.
Units that don't have built-in Auto Chart Live can do that also.