On the computer which you use to connect to the server core, run this:
netsh advfirewall firewall set rule group=”Remote Volume Management” new enable=yes
Source: http://social.technet.microsoft.com/Forums/windowsserver/en-US/22ca1d25-05b5-416c-bf9b-b301a2065d31/forum-faq-when-you-use-disk-management-to-connect-to-a-remote-computer-in-the-same-domain-you?forum=winserverfiles