Transmission and usb hard drive

Hello everyone, and thank you to begin your work and integration of transmission in apstore. My English is not great, I wrote with google translate. Hope this is understandable.

I’m stuck in the transmission configuration, I also use gui remote transmission. I can not find the way to my destination specified folder.
I connected a hard disk 4TO Mybook usb on my raspberry pi 2 to store and retrieve my files.
mybook ----> torrent
Can you tell me the way I need indicated in the remote gui or putty if not possible
thank you in advance for your assistance