I just figured out today that as long as the battery is in my laptop, it doesn't matter if it's fully charged while plugged in, Ubuntu always spins down my hard drive. I noticed this because there was a huge difference in speed when I removed the batteries. 

My settings for power management is basically:

* on AC power, don't spin down
harddrive, dont suspend or anything
* on battery power, basically save as
much power as possible

I assumed that if I plug in my laptop, it'll use the On AC Power settings no matter what but apparently, this isn't so. Is there a way to "fix" this?