I've got one of these drives as well, bought it new about 2 1/2 years ago now and it's a dog.
It does exactly what you describe - spins the drives down to 'save' power and then when your computer needs access, they take so long to spin back up that the computer often decides they have been lost and unmounts them.
I did a lot of searching back when I first got them and found a Western Digital support doc saying that it was the way the unit was built and that there was no way to change the sleep settings.
On the plus side, with their hefty weight they serve a second life as great book-ends.
EDIT - A techie friend of mine wrote a little script to touch the HDD every 2 minutes to keep it active which worked fine, except that the drives now got so hot that the fan went to high constantly and sounded like a jet engine.
Like I said, a dog of a drive.