pb files sharing on usb drive

Hello! I've installed the beta 2 (build 53.84) on my laptop.I can share files which are on my hard drive(by right clicking,sharing) and all is OK (a friend can read files...) but when I do the same with my external usb drive, my friend sees the pc and the folder but he can't access to the files inside..."not enough memory on the server or something like that"
if someone has an idea. thanks
sorry for english (I'm a french guy)