MY HDD is reading something, but I have no idea what. Is there an app I can use to monitor processes which read/write to HDD and where they read/write?
Yes, there is:
Press Ctrl+Shift+ESC; Go to "Performance" menu; Go to "Resource Monitor"; It will popup another windows. On this window select "Disk".
This is where you can see which process are using more or less disk I/O operations.