I haven't tested "mounting" yet, but when attempting to unmount an existing VHD, I receive an error:
COMMAND: VMOUNT64.exe DETACH C:\Secure.vhdx
STDOUT: OpenVHD failed,1392
COMMAND: VMOUNT64.exe UMOUNT S:\
Are you saying that you attempted to UNmount a NON mounted VHDx?
Then what is corresponding to the S:\ drive?
HOW (which program/driver/whatever) made the C:\secure.vhdx be accessible as drive letter S:\ ?
Do you know that a .vhd is NOT a .vhdx (they are DIFFERENT formats)?
Are you running Windows 7 (which knows nothing about .vhdx) or Windows 8/8.1 (which has "native" support for both .vhd and .vhdx)?