By default Disk metrics in Task Manager are not monitored on Server 2012 due to performance implications. Monitoring Disk Metrics is a useful when troubleshooting issues.
Task Manager (Default Settings)
To enable Task Manager’s Disk Counter:
Open PowerShell and use the following cmd DiskPerf
To enable DiskPerf you will need to use the Switch DiskPerf -Y (Yes)
To disable DiskPerf, Use DiskPerf -N (No)
Ryan Mangan's IT Blog 
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