problems with SD card

Writing to SD card creates strange results.
Sometimes file is created several times.
Sometimes file holds strange characters.
Sometimes readdir returns strange characters.
Please see attached screenshot.

var fs = require("fs");
var i = 0;
fs.appendFile("Juergen.json","hugo\n");
setInterval(function(){
  digitalWrite(LED1,true);
  fs.appendFile("Juergen.json","emilio");
  setTimeout(function(){digitalWrite(LED1,false);},200);
  i++;
},2000);

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