It is indeed your hard drive activity light.

Hard drive can go to work if Windoze tasks it with some background chore.

Another reason for the activity can be due to what is called temperature compensation. You see, the heads that read the magnetic disks are so close to the disk surfaces that it is not good to leave them sit in one place too long. So the designer built a bit of code into the disk drive circuits that will move the headarm around for a bit after it has sat in one place for awhile. Benign, and normal operation.


--Mac