Windows 7 Bug? HD Spindown

I think hard drive spin down is not working correctly in all instances. Can someone else confirm this?

I have 5 hard drives. If I set them to spin down after 45 minutes of inactivity, they do. I can hear them spin down, and for that matter, I can hear them spin up when I go to use them (and of course the 5 second or so delay when opening the drive)

Now, If I set them to spin down after 60 minutes, they never seem to spin down. Going to bed and waking up, I can access any drive and they open instantly, I can open files, everything. It seems to me they never spun down when set to 60 minutes.

EDIT: Windows 7600 RTM

I had this when I was running 5 drives in my machine.

I removed two, and no more problem. I never went as far as altering their spin down settings though.