A power operating system for opening and closing a vehicle liftgate has a pair of drive units supported on the vehicle roof and connected to the liftgate for opening and closing the liftgate. Each drive unit includes a bracket that is secured to the vehicle body for supporting several parts including a reversible electric motor, a gear unit and a track. The electric motor drives a segmented drive linkage inside the track via a gear reduction unit and a sprocket. The segmented drive linkage includes an elongated arcuate link arm that slides in an arcuate track portion of the track and a power roller chain that is stored in a storage track portion when the liftgate is closed. The outboard end of the link arm is pivotally connected to the liftgate to open and close the liftgate as the link arm is extended and retracted by the power roller chain.