files do not seem to like me :-(

Sorry for bothering you - but I am completely unable to get even simple file operations working.

The following code

const Storage = require('Storage');
  function compareFileWith (expectedContent) {
    if (Storage.read('4testing') != expectedContent) {
      print('>>>> file does not look as expected');
      print('>>>> expected "' + expectedContent + '"');
      print('>>>> got "' + Storage.read('4testing') + '"');
    }
  }

/**** create file ****/

  let TestFile = Storage.open('4testing','w');

  TestFile.write('Hello, World!');
  compareFileWith('Hello, World!');

/**** read file ****/

  TestFile = Storage.open('4testing','r');

  let readText = TestFile.read(5);
  if (readText != 'Hello') {
    print('>>>> reading "4testing" failed');
    print('>>>> expected "Hello", got "' + readText + '"');
  }

  TestFile.read(2); // skip ', '

  readText = TestFile.read(5);
  if (readText != 'World') {
    print('>>>> reading "4testing" failed');
    print('>>>> expected "World", got "' + readText + '"');
  }

  TestFile.read(2); // skip '!' and go beyond EOF
  print('read after EOF: "' + TestFile.read(4) + '"');

/**** write file ****/

  TestFile = Storage.open('4testing','w');

  TestFile.write('Welcome');
  compareFileWith('Welcome');

  TestFile.write(', World!');
  compareFileWith('Welcome, World!');

/**** extend file ****/

  TestFile = Storage.open('4testing','a');

  TestFile.write('\nHere I am!');
  compareFileWith('Welcome, World!\nHere I am!');
print('finished');

produces this output

read after EOF: "undefined"
>>>> file does not look as expected
>>>> expected "Welcome"
>>>> got "Hello, World!"
>>>> file does not look as expected
>>>> expected "Welcome, World!"
>>>> got "Hello, World!"
>>>> file does not look as expected
>>>> expected "Welcome, World!
Here I am!"
>>>> got "Hello, World!"
finished

which basically means that an existing file can not be overwritten using Storage.open(filename,'w') followed by file.write(newcontent).

Does anybody see what I'm doing wrong?