I’m having the same problem. As are may others ( http://community.wdc.com/t5/Networking/Can-t-access-network-share-feature-consistently/td-p/5215
Its working at the moment but I’ve fixed it in the past and then its stopped working again!
Check out http://www.daniloelectronica.com/windows-7/enable-network-shares-on-wd-tv-live-windows-7-ultimate-x64.html for a good overall tutorial.
You’ll also need to check the node type of the PC who’s files you are accessing is set to Broadcast. You can check by running a ipconfig /all in a dos box. It’s probably set to Hybrid, you should update it to Broadcast.
Check out http://forums.techguy.org/networking/161619-how-change-node-type-broadcast.html for instructions.
Let me know how it goes!
