Every network device does have an IP address associated with it, that's normal. However, that doesn't give you filesharing capabilities unless there's a software service on the device to allow you to do so. The 360 doesn't and it's impossible to add one. I understand what you're trying to accomplish, but sometimes unfortunately the answer is "it can't be done", and that is the case here. That's why I suggested the streaming alternative. I understand that streaming has some limitations as well, but in your case, it's the only way to allow simultaneous access to the drive from the PC and the 360.
Wish I had better news for you. Hope the info helps!