A method of capturing user control inputs includes providing a first navigation member slidably coupled on a housing portion of a portable computer and a stationary second navigation member external to and independent of the portable computer. An input card is provided that includes an optical navigation sensor and is configured with a size and a shape for deployment of the input card in a first releasably fixed arrangement relative to the first navigation member and a second movable arrangement relative to the stationary second navigation member. A first user control input is captured for controlling a function of the portable computer, via communication between the input card and the portable computer, based on relative motion between the optical navigation sensor of the input card and one of respective movable first navigation member and stationary second navigation member.