Disk activity?

Hey, on my 1000 Pro I have noticed that there is almost always constant disk activity.
I can hear the internal HDD churning away almost all the time, even when screensaver is on and I haven't done anything on the unit for a long time.

What is causing this (can I see that somewhere/somehow) and how can I control it?
I have "Automatic scan sources" set to OFF, "Automatic updating sources" to ON