I had this problem recently with the GF's W10 laptop. I couldn't figure it out.
Task manager would report 100% disk usage, but none of the processes would be anywhere near 100%. I tried disabling services, windows search, "shop me tips about windows 10" notifications. Loads of things. No luck.
I eventually did a fresh install.
That's the next step.
I've turned off what I can etc and it spikes to 100% for a couple of mins on load up or if you login from locked screen but then settles down.
I've told her to use it as normal, if it carries on, then it's fresh install time. Problem is it's one of the laptops the old IT guy updated to win 10. They've no records of serials etc... :'(