From what Zodiark1593 is saying, it seems the drive spinning down is the larger cause of access delay, not the head parking. Can you totally prevent a WD Green from spinning down using normal Windows power management, or will it do that by default similar to my external Seagate?
I've been factoring reliability into it, but with the uncertainty of it all, it can't be the sole factor. It seems WD Red is another option, if you want WD but faster than Green and cheaper than Black. I guess it's 5400 RPM and doesn't park its heads as often.