That feeling when you fix your 100% disk usage.
P.S. It was Superfetch AND "Microsoft Compatibility Telemetry"

I discovered the solution a month ago, I'm so happy now!

@BrianValente What was it for you?

I just renamed CompatTelRunner.exe (That's Microsoft Compatibility Telemetry) to CompatTelRunner2.exe so it can never run. It's not an important process, just Microsoft's spyware.

@randomCoder I upgraded to Windows 10 Education with college's free license, disabled telemetry using Group Policy Editor (this policy only works on Enterprise and Education), disabled Superfetch, Windows Search (only if the machine has an SSD) and Windows Update (enabling manually when I want to update).
As an extra you can use Spybot Anti-Beacon, but I think it's not necessary.