A horizontal directional drilling system is used to drive operation of a guidable reamer assembly connected to a drill string. The guidable reamer assembly preferably has a cutting member with a central longitudinal axis and a support member also having a central longitudinal axis. The longitudinal axes of the cutting member and the support member are collinear when the reamer assembly is in the non-steering position and laterally displaced when in the steering position. The assembly and method of this invention provide for increased control of reaming operations and product pipe placement.