We have a RNS310 fitted but it has no maps and I'm confused about how to get navigation working.

Lookinh on eBay I can see what look like 2nd hand CDs and SD cards.

I'd have though the SD cards woukd be locked to the unit they were previously used in, so am wary of getting those.

I expect the CDs to not be locked, so a better bet. If I put a large SD card in does the map copy to that SD, leaving space for me to use for music?

Is there an approach that I'm missing?

TIA