I think u should consider combining D, E and your new drive into a RAID 0 array for your media. Even with these three together you don't have a lot of bandwidth for some formats but it will be better than having just two drives in the raid.
As a general rule don't use Variable spin rate or lower rpm drives for media. The latency on mechanical drives is already much higher than SSD's and high PWM drives add to that. Green drives are meant for archive or sample storage and that is about it. I would suggest making the raid 0 out of the 2 Black drives for now and move to an SSD media drive when you are working with raw. The Crucial 960GB drives are really inexpensive for size at release. Testing with 4x 960GB on a Intel mid level SAS controller gives 1.8GB/s to 3GB/s depending on the file sizes.