I have the same problem.  i just got my nexus yesterday and i want to upload some pictures i have on my comp to the N7.  I used the USB attachment route and opened up Computer and then the N7 folder.  However, the simple drag and drop technique isn't working. 

***edit***
nevermind...i dont know why but all of a sudden it started allowing me to transfer. i have no idea why...i did nothing but let it sit on the plug for over 30 minutes. but hey...it works haha.