Hard drive wakes up when focussing Finder after some time

Hi there,

I am running Lion on a Mac Pro. And have a strange thing, not sure if it occured since installation, or if it popped up only more recently.

Everytime I access a Finder window after working for some time on other application windows, I hear my computer "awakening" and it takes 20-30 seconds until the Finder is active, while the colored wheel is turning and nothing happens. It seems as if Finder is sitting on another drive (I have several of them, but the OS is all on one).

Any hint what could cause this and how I could resolve it?

Thanks for any advice!