First init.

libraries/SD/examples/ReadWrite/ReadWrite.ino

@@ -0,0 +1,80 @@
+/*
+  SD card read/write
+
+  This example shows how to read and write data to and from an SD card file
+  The circuit. Pin numbers reflect the default
+  SPI pins for Uno and Nano models:
+   SD card attached to SPI bus as follows:
+ ** SDO - pin 11
+ ** SDI - pin 12
+ ** CLK - pin 13
+ ** CS - pin 4 (For For Uno, Nano: pin 10. For MKR Zero SD: SDCARD_SS_PIN)
+
+  created   Nov 2010
+  by David A. Mellis
+  modified  24 July 2020
+  by Tom Igoe
+
+  This example code is in the public domain.
+
+*/
+#include <SD.h>
+
+const int chipSelect = 10;
+File myFile;
+
+void setup() {
+  // Open serial communications and wait for port to open:
+  Serial.begin(9600);
+  // wait for Serial Monitor to connect. Needed for native USB port boards only:
+  while (!Serial);
+
+  Serial.print("Initializing SD card...");
+
+  if (!SD.begin(chipSelect)) {
+    Serial.println("initialization failed. Things to check:");
+    Serial.println("1. is a card inserted?");
+    Serial.println("2. is your wiring correct?");
+    Serial.println("3. did you change the chipSelect pin to match your shield or module?");
+    Serial.println("Note: press reset button on the board and reopen this Serial Monitor after fixing your issue!");
+    while (true);
+  }
+
+  Serial.println("initialization done.");
+
+  // open the file. note that only one file can be open at a time,
+  // so you have to close this one before opening another.
+  myFile = SD.open("test.txt", FILE_WRITE);
+
+  // if the file opened okay, write to it:
+  if (myFile) {
+    Serial.print("Writing to test.txt...");
+    myFile.println("testing 1, 2, 3.");
+    // close the file:
+    myFile.close();
+    Serial.println("done.");
+  } else {
+    // if the file didn't open, print an error:
+    Serial.println("error opening test.txt");
+  }
+
+  // re-open the file for reading:
+  myFile = SD.open("test.txt");
+  if (myFile) {
+    Serial.println("test.txt:");
+
+    // read from the file until there's nothing else in it:
+    while (myFile.available()) {
+      Serial.write(myFile.read());
+    }
+    // close the file:
+    myFile.close();
+  } else {
+    // if the file didn't open, print an error:
+    Serial.println("error opening test.txt");
+  }
+}
+
+void loop() {
+  // nothing happens after setup
+}