The relevant portion of my test code is below; apologies if it comes through with weird formatting:

if (!SD.begin(4,SPI,40000000)) { // ******M5 CS line is GPIO4
Serial.println("initialization failed!");
while (1);
}
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.

NewFile = SD.open("/TRIAL.TXT",FILE_WRITE);  
char message1[64]="This is just a test";  // 64 byte array
char message2[64]="It seems to be working so far!";

// delay(5000);
// if the file opened okay, write to it:
if (NewFile) {
NewFile.println(message1);
NewFile.println(message2);

// close the file:
NewFile.close();

} else {
// if the file didn't open, print an error:
Serial.println("error initially opening test.txt");
}

// Now try reopening file and writing more data into it

 delay(100);
 NewFile = SD.open("/TRIAL.TXT",FILE_WRITE);  //*******Reopen for more writing
 NewFile.println("Trying to add this additional line");
 NewFile.close();
 delay(100);