error opening textfile on adafruit sd-breakout using arduino pro mini

[system]

i am currently working on a school project. i need to control a torch and read out the progression of the voltage using three different measurements. i'm logging the data from the measurement on a adafruit.com 5V ready micro-sd breakout board.
before i'll explain to much i'm just gonna post the code which i have so far:

#include <SD.h>
const int chipSelect = 4;
const int akkuA = 9;
const int akkuB = 8;
String dataString;

//----Internal Readconfig----
long readVcc() {
  long result;
  // Read 1.1V reference against AVcc
  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
  delay(2); // Wait for Vref to settle
  ADCSRA |= _BV(ADSC); // Convert
  while (bit_is_set(ADCSRA,ADSC));
  result = ADCL;
  result |= ADCH<<8;
  result = 1126400L / result; // Back-calculate AVcc in mV
  return result;
}


//---------------------------
// setup
//---------------------------
void setup() {
  Serial.begin(9600);
  analogReference(INTERNAL);
  analogRead(0);
//  pinMode(13,OUTPUT);  //evtl löschen???
  
//---SD-----
  Serial.print("Initializing SD card...");
  pinMode(10,OUTPUT);
  if(!SD.begin(chipSelect))  {
    Serial.println("Card failed, or not present");
    return;
  }
  Serial.println("card initialized");
  

}

//---------------------------
// loop
//---------------------------
void loop() {
  
  
//----Messungen----
  readVcc();
  int sensA = analogRead(A1);  
  double voltA = sensA * (5.0 / 1023.0);
  Serial.println(voltA,3);

  readVcc();
  int sensB = analogRead(A2); 
  float voltB = sensB * (5.0 / 1023.0);
  Serial.println(voltB,3);
  
  readVcc();
  int sensC = analogRead(A3); 
  float voltC = sensC * (5.0 / 1023.0);
  Serial.println(voltC,3);
  
//----Spannung über Widerstand----
  Serial.print("Diff:");
  float diff = voltB-voltA;
  Serial.println(diff,4);


//----Umwandlung + Zusammenfügen----
  char spanA[5];
  dtostrf(diff,1,3,spanA);
  
  char spanB[5];
  dtostrf(voltC,1,3,spanB);
  
  
//----dataString zurücksetzen + Werte speichern----
  dataString=""; //nötig?
  dataString = String(spanA) + "," + String(spanB);

  
//----Speicherung-----
  File dataFile = SD.open("datalog.txt", FILE_WRITE);

  if (dataFile) {
    dataFile.println(dataString);
    dataFile.close();
    Serial.println(dataString);
  } 

  else {
    Serial.println("error opening datalog.txt");
  }
  
  
//----Laden1?---- // 3.3V=675.18 / 3.7V=757.02
  if (sensA > 680 || sensA < 755) {
  digitalWrite(akkuA,HIGH);
  }
  else {
  digitalWrite(akkuA,LOW);
  }
  
//----Laden2?---- // 3.3V=675.18 / 3.7V=757.02
  if (sensA > 678 || sensA < 755) {
  digitalWrite(akkuB,HIGH);
  }
  else {
  digitalWrite(akkuB,LOW);
  }


  
//----Delay zwischen Messungen
  delay(1000);
}

this is what it shows when i open the serial monitor:

Initializing SD card...card initialized
1.036
1.095
1.036
Diff:0.0587
0.0591.036,1.036Î

1.056
1.065
0.958
Diff:0.0098
0.0100.958,0.958ý

1.046
1.065
0.948
Diff:0.0196
error opening datalog.txt

after three iterations it resets and starts over in the same pattern. i dont understand why it finds the sd-card but cannot open the datalog.txt. i've already formatted my sd using the sdformatter and tried different unicodes (UTF-8 and UTF16) but neither works. after running the program and plug my sd back to my mac it says that my mac isn't able to open datalog.txt because it's not unicode or it shows my unreadable signs

i've also tried the chipSelect variable on 10 (on the code it's 4) but it doesnt work either. the only thing that currently works is the datalogger from the library. note that i read that the string dataString =""; should not be in the loop since it's not necessary that it creates a new variable each iteration so i moved it up to the top of the code.

does anyone see an error?

[system]

  dtostrf(diff,1,3,spanA);

One digit before the decimal point, the decimal point, three characters after the decimal point, and terminating NULL will not fit in a 5 element array. You lied to this function when you said the array was big enough.

[system]

i need to control a torch

TORCH=FLASHLIGHT

[system]

thanks PaulS. that solved the problem

i got another one though. when i run it on the USB-Port it shows more or less the exact value. but as soon as i unplug it form the computer and run it with the battery the output is only 0V (the difference) and 5V as the voltage from the battery, which is actually only 3.7V.

i thought by using

long readVcc() {
  long result;
  // Read 1.1V reference against AVcc
  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
  delay(2); // Wait for Vref to settle
  ADCSRA |= _BV(ADSC); // Convert
  while (bit_is_set(ADCSRA,ADSC));
  result = ADCL;
  result |= ADCH<<8;
  result = 1126400L / result; // Back-calculate AVcc in mV
  return result;
}

and   analogReference(INTERNAL); it compares the measured voltage to the internal 1.1V

what am i doing wrong this time?