if(SD.exists("data.csv"))
This is wrong, because a file was never created in setup. You are creating the file the first time when you open it in loop().
This works
void saveData(){
//if(SD.exists("data.csv")){ // check the card is still there
// now append new data file
sensorData = SD.open("data.csv", FILE_WRITE);
if (sensorData){
sensorData.println(dataString);
sensorData.close(); // close the file
}
else{
Serial.println("Error writing to file !");
}
}