WD TV Live wont play videos off MyBook Live

My WDTV Live has stopped playing videos from my WD MyBook Live video folder. It worked fine until a few days ago. I can see the video folder, and the files inside it but when I try to play one all I get is the little circle going round. If I copy the file to a memory stick, WDTV will play it ok from that, so it’s not a corrupt file. I’ve rebooted everything but that made no diffference. Any ideas?

How are you accessing the file?  Samba?  Media Server?