It has happened 2 days. RAM usage is always 100%. Resource Monitor found that wd file management engine use the rest of RAM. I have 4 GB of Ram and it use 1.8GB. It slow down my PC.
I also found out that "svchost.exe (LocalServiceNetworkRestricted)" use all of harddisk bandwidth at the same time.
Now I cope with problem by stop the "wd file management engine" process.
What should I do in the long term? How did it happend?