I was able to write a float into the SD card. I used the ReadWrite sketch from the SD examples. Used the myFile.println(); function to write random floats in the SD card and it worked fine. Maybe you can try this to see if it works for you.
ok thank you i have tried a another sketch: first convert double to string and than write it to the sd card so i try it now with the other example i hope it works with this :-)