I've run into the same problem as you trying to share a drive (internal) of the system. What I've found is that this is inherent to Windows, all windows platforms, and designed this way for security reasons and protection of the system. Because if you allow access to a drive that has system files needed for windows to operate, then a user with access can change/modify said files which can render the system inoperable. Not saying this can't be done, but it's a long drawn out process to allow certain users, parameters for each user and so on...and just a real pain...for me anyway.
What works and much easier, is get an HDD inclosure and install drives that you want to share and hook that up directly to the router, via usb or eSata, which...