I googled and the first result is really bad (it is a blog from http://www.modmypi.com). I tried to follow it and it gave me a lot of troubles and it didn’t really mount the drive. I wrote all my data on my pi and I had to clean it up.

The correct steps are here:
http://www.techjawab.com/2013/06/how-to-setup-mount-auto-mount-usb-hard.html