The underscore character is allowed in the system name as long as NetBIOS is running in the network, but I believe is n longer allowed as of Windows Server 2008 R2 and later, as WINS/NetBIOS is no longer available. The underscore is not supported by DNS, only dash, and 2008R2 relies on DNS-integrated Active Directory.
The workstations support it because Windows 7 will still utilize NetBIOS and WINS if it is present in the network; Windows 8 will not.
Western Digital follows these guidelines in their system setup and does not allow you to use an underscore character in the system name.
When naming systems in the future, you should adhere to the DNS naming guidelines, and if possible, rename your existing workstations and servers to remove the underscore character, otherwise you may experience problems when upgrading the functional level of your domain.
