According to the documentation on Technet, the /MT
parameter applies to Server 2008 R2 but not Server 2008. I'm not sure if this is due to just the version of the robocopy executable or some other restriction.
What is the File Version of your robocopy.exe, as shown in the file properties?
You might try to pull the newest version of Robocopy (6.1.7601) out of KB2639043 and see if that works for you. Though, information I found on this thread suggests that newer versions of Robocopy don't work at all on Server 2008.