D1 Mini Micro SD Card Module

/*
Here's an example on how to use a Micro SD card module with a Wemos D1 Mini (ESP8266) to read and write data to a text file. This example uses the SD and SPI libraries that come with the Arduino IDE.

Connect the Micro SD card module to your Wemos D1 Mini as follows:

VCC to 3.3V
GND to GND
MISO to D6
MOSI to D7
SCK to D5
CS to D8
Upload the complete code to your Wemos D1 Mini and open the Serial Monitor. The program will create a file called "test.txt",
*/

#include <ESP8266WiFi.h>
#include <SPI.h>
#include <SD.h>

#define CS_PIN D8

void setup() {
  Serial.begin(115200);
  Serial.println("Initializing SD card...");

  if (!SD.begin(CS_PIN)) {
    Serial.println("Initialization failed!");
    return;
  }
  Serial.println("Initialization successful.");
}

void loop() {
  File dataFile = SD.open("test.txt", FILE_WRITE);
  if (dataFile) {
    dataFile.println("Hello, world!");
    dataFile.close();
    Serial.println("Data written to test.txt");
  } else {
    Serial.println("Failed to create test.txt");
  }

  dataFile = SD.open("test.txt");
  if (dataFile) {
    Serial.println("Reading data from test.txt:");
    while (dataFile.available()) {
      Serial.write(dataFile.read());
    }
    dataFile.close();
  } else {
    Serial.println("Failed to open test.txt for reading");
  }

  if (SD.remove("test.txt")) {
    Serial.println("test.txt has been deleted");
  } else {
    Serial.println("Failed to delete test.txt");
  }

  delay(5000);
}