When a disk rotates at constant angular velocity in otherwise still fluid a boundary layer flow is established close to the disk. Obviously the local velocity of the disk increases with radius, but to a first approximation, this radial variation of the basic flow can be neglected to produce a "local" parallel-flow linear stability theory. The local stability theory predicts that the flow becomes absolutely unstable beyond a certain radius. However, it was found that the full radially-dependent "global" stability theory predicts global decay of disturbances, contrary to the local results, and more recent experiments appear to confirm this global decay. In this paper we present a new mechanism for global instability of this flow, and show that it correctly predicts the qualitative behaviour of laminar-turbulent transition observed in experiments.