Hard drive will not sleep automatically but does manually

Hey Folks,
I've got a 15" macbook pro (2009) in which I upgraded the hard drive from 250 to 500GB Western Digital Scorpio Blue HD.
My problem is that it will not go to sleep on its own. The monitor goes blank and the sleep light comes on full (not blinking), but the hard drive continues to run.
If I manually select sleep or close the lid, the hard drive shuts down and the laptop is fully asleep, but it will not automatically sleep.
I took it to the "Genius Bar" yesterday and the tech suggests that it's the hard drive. via the following troubleshooting steps.
He booted the laptop from his external system drive. While my hard drive was installed it still would not automatically go to sleep (full light only not blinking light), but when he removed my hard drive the laptop would automatically go to sleep (blinking light).
Now I don't argue that this does seem to indicate that the hard drive has something to do with it, but I don't understand why I can manually put it to sleep via the apple menu but it will not do this automatically. It's as if the scripted command isn't the same as the apple menu command.
If the conclusion is I need a new hard drive, I really don't mind too much as I see there are shiny new 1TB drives out there at a reasonable price, but I want to be sure that a new drive will not behave the same.
Any suggestions??

Have you tried going into System Preference under 'energy saver' it has an option to 'put hard disk to sleep when possible'

Yes, the put to sleep when possible is checked.

I would contact WD for help. I'm thinking the problem is on their side. I have a Hitachi Travelstar 7200. No problem so far.