This is because disk metrics are disabled by default on Windows Server 2012 because of significant performance impact, however, it is enabled in Windows 8.Use Resource Monitor to gauge Disk and Network IO usage. - Syed Yusuf from Microsoft R&D
If you need to see the metrics, you can use Resource Monitor.
Alternatively:
You can enable Disk performance monitoring on this tab by running the following command from a Command Prompt launched using the "Run as Administrator" option.
