An ex-situ Servo Track Writer (STW) uses a support element that can extend between discs in a stack, and can also retract, permitting a high level of variation in the stack's positioning. The support element preferably has an engagement surface that is wide enough to permit the element to support the actuator throughout the element's range of (rotary) motion. Because the support structure is retractable, it can use low angles of approach like those of hyperbolic-shaped cams, without losing access to the outermost portions of the discs. The support structure may therefore be moved out of the servowriter actuator's path while position data is written to the outermost portions of the data surface.