Do you ever leave the laptop on, plugged in to AC, just idle for an hour? Usually constant hard drive usage indicates Programs running (you checked those), processes running, system processes and such which perform maintenance and updates, and last but not least viruses and malware.
So try this, shut down completely, remove battery if possible, power up and reboot, log on and leave the laptop alone for 60 minutes and periodically check to see if the system settles down, run a virus scan, malware scan. Then you may need to use resource monitor to find what process or program is using your disk. Should be around 5% max at idle with no programs running