This problem is caused by the “Secure Negotiate” feature that was added to SMB 3.0 for Windows Server 2012 and Windows 8. This feature depends upon the correct signing of error responses by all SMBv2 servers, including servers that support only protocol versions 2.0 and 2.1. Some third-party file servers do not return a signed error response. Therefore, the connection fails.
You can read the Microsoft KB article here: http://support.microsoft.com/kb/2686098
See my post here http://forum.mikrotik.com/t/feature-request-smb-v3-or-proper-v2-support/78393/1 and another post here with the solution http://forum.mikrotik.com/t/rb951ui-smb-not-share/77304/1
The solution involves disabling secure signing which in my opinion isn’t really a solution. The issue is with MikroTiks implementation of SMB.