First init.

libraries/SD/examples/NonBlockingWrite/NonBlockingWrite.ino

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+/*
+  Non-blocking Write
+
+  This example demonstrates how to perform non-blocking writes
+  to a file on a SD card. The file will contain the current millis()
+  value every 10ms. If the SD card is busy, the data will be dataBuffered
+  in order to not block the sketch.
+
+  If data is successfully written, the built in LED will flash. After a few
+  seconds, check the card for a file called datalog.txt
+
+  NOTE: myFile.availableForWrite() will automatically sync the
+        file contents as needed. You may lose some unsynced data
+        still if myFile.sync() or myFile.close() is not called.
+
+  Pin numbers reflect the default SPI pins for Uno and Nano models
+  Updated for clarity and uniformity with other examples
+
+  The circuit:
+   analog sensors on analog ins 0, 1, and 2
+   SD card attached to SPI bus as follows:
+ ** SDO - pin 11
+ ** SDI - pin 12
+ ** CLK - pin 13
+ ** CS - depends on your SD card shield or module.
+    Pin 10 used here for consistency with other Arduino examples
+    (for MKR Zero SD: SDCARD_SS_PIN)
+
+    modified 24 July 2020
+    by Tom Igoe
+
+  This example code is in the public domain.
+*/
+#include <SD.h>
+
+const int chipSelect = 10;
+
+// file name to use for writing
+const char filename[] = "datalog.txt";
+
+// File object to represent file
+File myFile;
+// string to buffer output
+String dataBuffer;
+// last time data was written to card:
+unsigned long lastMillis = 0;
+
+void setup() {
+  // Open serial communications and wait for port to open:
+  Serial.begin(9600);
+  // reserve 1 kB for String used as a dataBuffer
+  dataBuffer.reserve(1024);
+
+  // set LED pin to output, used to blink when writing
+  pinMode(LED_BUILTIN, OUTPUT);
+
+  // 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.");
+
+  // If you want to start from an empty file,
+  // uncomment the next line:
+  //  SD.remove(filename);
+  // try to open the file for writing
+
+  myFile = SD.open(filename, FILE_WRITE);
+  if (!myFile) {
+    Serial.print("error opening ");
+    Serial.println(filename);
+    while (true);
+  }
+
+  // add some new lines to start
+  myFile.println();
+  myFile.println("Hello World!");
+  Serial.println("Starting to write to file...");
+}
+
+void loop() {
+  // check if it's been over 10 ms since the last line added
+  unsigned long now = millis();
+  if ((now - lastMillis) >= 10) {
+    // add a new line to the dataBuffer
+    dataBuffer += "Hello ";
+    dataBuffer += now;
+    dataBuffer += "\r\n";
+    // print the buffer length. This will change depending on when
+    // data is actually written to the SD card file:
+    Serial.print("Unsaved data buffer length (in bytes): ");
+    Serial.println(dataBuffer.length());
+    // note the time that the last line was added to the string
+    lastMillis = now;
+  }
+
+  // check if the SD card is available to write data without blocking
+  // and if the dataBuffered data is enough for the full chunk size
+  unsigned int chunkSize = myFile.availableForWrite();
+  if (chunkSize && dataBuffer.length() >= chunkSize) {
+    // write to file and blink LED
+    digitalWrite(LED_BUILTIN, HIGH);
+    myFile.write(dataBuffer.c_str(), chunkSize);
+    digitalWrite(LED_BUILTIN, LOW);
+    // remove written data from dataBuffer
+    dataBuffer.remove(0, chunkSize);
+  }
+}