How can I copy unmanaged data in C# and how fast is it?

I have two unmanaged pointers in the form of IntPtr and want to copy data between them. How can I do this? I know the method Marshal.Copy, but it can only copy between unmanaged and managed. And the second part: Is copying unmanaged data from C# slower than doing it in unmanaged C/C++ using memcpy?

Edit: I would be especially interested in a platform independet implementation.