The 598 will only allow you to record or save anything if you remove the Navionics card and put a formatted data card in its place. Its certainly possible, but its a pain because you loose your pretty maps and depth lines, etc from the Navionics card while you are recording. You will be stuck with what ever level of detail the built in base map has for the area you are in.
Thats one of the major downsides to buying the cheaper models. To do both at the same time you need to up-grade to a unit with two sd-card slots = more $$.