I am connecting from a Dell XPS630 to a Dual 2.5 Ghz G5 tower.  My Life (Pictures, Movies, TV shows, etc) is on a Drobo Connected via USB.  I have no problems.  I am streaming to media center. I will tell you I am using all the drivers provided by windows because if I install the latest nvidia drivers for the Dell Motherboard and ethernet, I have to roll back the system because of the blue screen! But with Windows update drivers-Perfect!
I suspect a driver issue on your Mac.
I have not replaced the Bootcamp XP on my macbook Pro with win 7 so Guess I'm no help.