Hello again,
We are trying to read 320kb txt file to a Micro SD card using an Teensy 3.6. I divided my file into 512 byte buffers in C# and every 65 milliseconds write it to SerialPort. But in Teensy same code goes infinity loop while starting serial (while(!Serial){}).When we replace Serial with Serial1 problem has solved but we could not write buffers to SD card now. Here is the code:

Code:
#include <SD.h>
#include <SPI.h>
char gelen[512];
File myFile;

// change this to match your SD shield or module;
// Arduino Ethernet shield: pin 4
// Adafruit SD shields and modules: pin 10
// Sparkfun SD shield: pin 8
// Teensy audio board: pin 10
// Teensy 3.5 & 3.6 & 4.1 on-board: BUILTIN_SDCARD
// Wiz820+SD board: pin 4
// Teensy 2.0: pin 0
// Teensy++ 2.0: pin 20
const int chipSelect = BUILTIN_SDCARD;
const int ledPin = 13;
void setup()
{
  pinMode(ledPin, OUTPUT);
  
 //UNCOMMENT THESE TWO LINES FOR TEENSY AUDIO BOARD:
 //SPI.setMOSI(7);  // Audio shield has MOSI on pin 7
 //SPI.setSCK(14);  // Audio shield has SCK on pin 14
  
 // Open serial communications and wait for port to open:
  Serial.begin(115200);
   while (!Serial) {
    digitalWrite(ledPin, HIGH);
    ; // wait for serial port to connect.
  }


  while (!SD.begin(chipSelect)) {
  
    
  }
  
  
  // open the file. 
  myFile = SD.open("test.txt", FILE_WRITE );
  
 
  
  
}
void loop(){
  if(Serial.available()>0)
  {
    Serial.readBytes(gelen,512);
    myFile.println(gelen);
  }
  else
  {
    myFile.flush();
  }
}
Thank you.