Hi, Bertho, and welcome. Given that you have many programs running continuously that may be in real time, it can be hard to pin down what's actually causing the high disk usage. Especially when the task manager shows nothing out of the ordinary.
You could try turning off Norton Background Tasks and also Performance Monitoring, which itself can cause occasional slowdowns.
That may help eliminate Norton 360, as causing high disk activity.
I assume scans have shown no malware detection ? You could always try scanning with MalwareBytes Free edition, as a passive scanner, and see if it picks up anything.
Let us know, if the problem persists.