600mL

it's an essential problem. I just wonder to buy N+2, but after read this thread....  I think twice, maybe it's ok to keep develop my project using N+ 

It works fine if you flush afterwards, eg:

        public void writeToFile(string data)
        {
            using (FileStream filestream = new FileStream(filename, FileMode.Append))
            {
                using (StreamWriter write = new StreamWriter(filestream))
                {
                    write.WriteLine(DateTime.Now + "," + data);
                }
            }
                        
            VolumeInfo.GetVolumes()[0].FlushAll();      // Force write to sd card
        }

In any case, even if you do not flush, the card will be written to eventually. I believe the data is buffered in memory and then written to when it reaches a certain level. I think this is to avoid too many short read/write operations to the SD Card which will probably impact on performance.