What process is accessing the hard drive?
Solution 1:
I like using fs_usage to see whats going on. A lot of information is going to go by quick, but you usually can see which app is doing stuff. For example, when idle, ill see safari going pretty crazy at times, plus spotlight, etc.
Solution 2:
Other thing you can try is the iotop tool.
Example usage would be
sudo iotop # requires root privileges
And more info can be found here
It give summary over the fs_usage which was mentioned by jmlumpkin.