I am looking at getting a 1 or 1.5TB HDD for external storage, and for working with LR photos. The idea is that I'll access the RAW files from the external HDD. I'm looking at the Western Digital green drives, but I understand they are a bit slower than some of the other drives. Right now, I have an older 20" iMac with only FW400. Will I notice any difference in speed using the WD Green drive vs. others? My guess is that the bottle neck is likely in the FW400, so I wouldn't know the difference. But, my plan is to move to a newer iMac in the fall. Would I notice a performance over FW800 in the WD Green drives vs, say, WD Black drive?