If you have DLNA turned on, test to see if your windows 7 can see the DLNA server.
I had this issue with my first DUO. The DLNA server was on, and the PC could see it, but the XBOX would find it rarely, and even in those cases, access to the files themselves was taking hours. I returned the device and replaced it with another DUO. Now the XBox and all of the other devices can see the DUO just fine.
