Data are not saved in the Arduino SD

Question

I have created a code that takes temperature measurement data and saves it in a variable. Now I would like to save the data in a .txt file inside an SD card connected to Arduino. The initialization of the SD works, but when it manages to access the file it gives me errors, writing me a set of strange symbols instead of the file name and it doesn't write inside the file. I tried a very similar code to write inside the SD and it works fine. What problem can it be? I enclose the part of the writing code on the SD and the photo with the output. Initialization part:

Serial.println(F("Intializing SD card"));
if(!SD.begin(4))
{
  Serial.print(F("Initialization failed"));
  while(!SD.begin(4))
  {
    Serial.print(F("."));
    delay(1000);
  }
}
else Serial.println(F("Initialization done"));

Writing part:

sdFile = SD.open("records.txt",FILE_WRITE);
Serial.print("Writing to ");
Serial.println(sdFile.name());
sdFile.print(" Temperature: ");
sdFile.print(tempC);
sdFile.print(" taken at: ");
sdFile.print(hour());
sdFile.print(":");
sdFile.print(minute());
sdFile.print(":");
sdFile.println(second());

sdFile.close();

Image with the strange output

Answer 1

I suspect you are opening the SD card before it has a chance to initialise, to fix this, implement a check at the start like so:

if(!SD.begin(8)){ // Here, 8 is the CS (chip select) pin of your SD card.
Serial.println("initialization failed!");
    while (1);
}

sdFile = SD.open("records.txt",FILE_WRITE);

if(myFile){ // If we opened our file successfully!
// Write to our card here

}
sdFile.close();

Does this fix your issue? Also, watch out for size limitations on how large the Arduino can read SD cards. This is usually 2GB for full-size cards, and 16GB for "micro" cards.

Additional documentation can be found on the arduino website, as well as examples here.