Arduino SD File Write Not Saving Data

In this tutorial, you'll learn about Arduino SD File Write Not Saving Data. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

Data written to an SD file is lost or the file is empty after writing.

Quick Fix

Wrong

File f = SD.open("data.txt", FILE_WRITE);
f.print("Hello");  // Data may not be flushed to card

File exists but is empty or truncated.

Right

#include <SD.h>

void setup() {
  Serial.begin(9600);
  while (!Serial);
  
  if (!SD.begin(4)) {
    Serial.println("SD init failed");
    return;
  }
  
  File f = SD.open("data.txt", FILE_WRITE);
  if (f) {
    f.println("Line 1: sensor data");
    f.println("Line 2: 42");
    f.flush();  // Force write to card
    Serial.println("Data written and flushed");
    f.close();
  } else {
    Serial.println("Failed to open file");
  }
  
  // Verify
  f = SD.open("data.txt");
  if (f) {
    while (f.available()) {
      Serial.write(f.read());
    }
    f.close();
  }
}

void loop() { }

Data written and flushed
Line 1: sensor data
Line 2: 42

Prevention

Always f.close() after writing — close() flushes the buffer. For important data during logging, call f.flush() periodicaly (every N writes). FILE_WRITE opens for appending (starts at end of file). For overwrite, open in FILE_WRITE then f.seek(0).

DodaTech engineers apply these same patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro for production IoT reliability.

FAQ