Slow HD video whilst editing
If you have a 8 core CPU I'd bet it is a fast one too, so that shouldn't be a problem. 16 GB is more than enough for HD, and DDR3 should ensure the speed.
The GPU shouldn't have much to say as long as it's not crap.

If you have 6 slow harddrives, it would be a bottleneck. You should have at least 7200 rpm disks!