I was working on a Windows Phone XNA example earlier and decided to run it on my Xbox. After duplicating the project as an Xbox 360 project I kept running into the same error that seemed to have no explanation.
<pre>Building content threw InvalidOperationException D3DERR_NOTAVAILABLE</pre>
It took a bit of time to figure out what was going on here. From doing a search on the Internet I found out that certain Direct3D programs cannot run at the same time as an XNA project. I don't fully understand why. But as it turns out the program that was causing me to experience this problem was the Zune client. Once I closed the Zune client I was able to compile and run my Xbox 360 program. Weird.