You cannot use but here. "But" implies that there is a counterpoint to what is said before the "but". Example: I waited at the train station, but my friend never came. "Except" implies that there is one thing (or group of things) that was singled out. Example: Everyone waited at the train station except Martha. It singles out a group or an individual.

acording to my grammar book the best answer is "except", "but" is used for contrasting two or more things,for example: She is very tired, but she has to go to her office. hope this can help you. :)