Correct going to/from the same drive will hurt unless you have another bottleneck that's masking it from you (like say, your cpu in the transcoding process can't process the data fast enough). And depending on your direction of converting (ie, mpeg-2 -> h.264/mpeg4) you want the faster drive to be the mpeg-2. So raid-0 on that side and perhaps a single drive on the h.264 side. At the drive level it doesn't really care what the data is, just the size of the request. In this case you're compressing something more so you'll take say 4x the request size coming in (mpeg-2) and are going to be writing out 1x (assuming 4:1 compression/recoding). So your output drive will be doing 1/4 the amount of work.