Starting with SP4, RDR.SYS was changed to fix a problem. This causes some additional network traffic.
Use Regedt32 to navigate to:
Edit or Add Value name FileSystemControlFilter as a type REG_DWORD. In the DWORD Editor, set the data value to 0.
The default is 1, which filters FSCTL_IS_VOLUME_MOUNTED.
This will be fixed when SP7 becomes available.
There is a current hotfix available, if you call Microsoft and mention fix