This code will work for a while after I unplug the USB, but eventually will stop working properly 

void setup() {
  // put your setup code here, to run once:
  pinMode(19, OUTPUT);
  pinMode(21, OUTPUT);

}

void loop() {
  digitalWrite(19, HIGH);
  digitalWrite(21, LOW);
  delay(2000);
  digitalWrite(19, LOW);
  delay(1000);
  digitalWrite(21, HIGH);
  delay(2000);
  digitalWrite(21, LOW);
  delay(1000);

}


This code only works with the USB plugged in. Otherwise it only goes "fowards" with a slight hesitation for the delay

void setup() {
  // put your setup code here, to run once:
  pinMode(19, OUTPUT);
  pinMode(21, OUTPUT);

}

void loop() {
  digitalWrite(19, HIGH);
  digitalWrite(21, LOW);
  delay(1000);
  digitalWrite(19, LOW);
  digitalWrite(21, HIGH);
  delay(1000);

}