Hard Drive Question

Hi All,

For a while now, about every 5 or 10 minutes, I'll hear a hard drive
in my computer sound like it's spinning up. Sometimes the drive
access light comes on, other times it does not. This happens very
frequently. Why is this? Is one of my drives constantly spinning
down? Can I prevent this? I'm running Vista.

Additionally, I have an external 1TB drive, connected via firewire.
It disappears from explorer every so often. I need to unplug the
cable and plug it back in........any thoughts on this??

Thanks!