Try reading it! There is a faq that explains how to do it.
" When I plug in my encrypted USB flash drive, Windows asks me if I want to format it. Is there a way to prevent that? Yes, but you will need to remove the drive letter assigned to the device. For information on how to do so, see the question 'I encrypted a non-system partition, but its original drive letter is still visible in the 'My Computer' list.' "