struct config_t {
    uint8_t foo;
    uint16_t bar;
};

uint8_t ptr[512];
struct config_t *config;

//read configuration from SD card
readSector(10 /*sector number*/, ptr);
config = (struct config_t*)ptr;

if (config->foo) {do something;}
config->bar = 35; //update some paramter

//save configuration to SD card
writeSector(10, ptr);