Won't Copy Files to USB Stuck at 0&

Hi

I have tried many times with two or three different USB drives but every time it gets to the point of “copying files” to the USB it gets stuck at 0%. This is frustrating as I want to FTP to my AppleTV to get the music off. Has anyone got any idea what I am doing wrong. I am a relative non techie so any help would be appreciated. 

Cheers

Adrian