This is my code:
Which could be greatly improved. Loose the String class completely. Open the file. Loop to read a sensor and write a value, with some data before and after, if desired, to the file. Close the file.
Some people have seen the need to delay for a short while after opening the serial port, before writing to it. That, generally, shouldn't be necessary, but the stuff printed to the serial port doesn't seem to come from this program, although the text is certainly present in the program.
What kind of Arduino are you using?