Just out of curiosity, you have turned off the ‘idle disks when possible’ preference pane option, haven’t you? It’s the simplest reason and I didn’t realize it until I discovered it last week – it can also apply to external RAID arrays.
That said, smaller files do tend to have a bit of an initial lower data rate. Under 1 gig on my AJA system test can show that. However, it’s not been a big issue for me yet. If you can make the lag show up on the test, perhaps post a screengrab and I’ll tell you if it’s far off from mine. I have noticed a few little weird things about these drives – something cold opening a finder window on it takes a second or two. Didn’t happen with 1TB Caviar Black drives…
A while back, I built an array from 1TB RE4-GP (Green Power) drives, which turned out to be totally not usable. There were massive delays in the drives which made them unusable. I wonder if WD is using that same power save tech with these drives?
UPDATE: I just installed an app called “SleeplessHD” off of macupdate that hits the drive every 5 seconds. Enable it at login and that should fix ya. It’s working for me now. We’ll see what happens….