|There are multiple causes for disk activity during idle periods, including:|
2. Windows update
3. AntiVirus scans or updates.
You can set when Windows Update and when AntiVirus updates and scans take place. Superfetch takes place during idle periods only so should not impact performance.
The System Idle Process isn't a real process but is used to show what CPU time is not being used by active processes. The higher the usage here the better.