Extremely high disk activity without any real usage

I had the same issue. I resolved it by changing the power plan from 'Balanced' to 'High performance'. Instant results.


Running the following command appears to have fixed the problem:

chkdsk /b /f /v /scan c:

Explanation

In the chkdsk version included with Windows 10 these flags mean:

/b NTFS only: Re-evaluates bad clusters on the volume (implies /R)

/f Fixes errors on the disk.

/v On FAT/FAT32: Displays the full path and name of every file on the disk.

/scan NTFS only: Runs an online scan on the volume

/r Locates bad sectors and recovers readable information (implies /F, when /scan not specified).

Thanks to @hennes for the inspiration.


Another reason for extremely HDD usage is MS Windows Defender. There is some antimalware service under Windows 8 that belongs to Windows Defender.