This paper uses Lyapunov’s method to determine the critical speed of a flexible spinning disk enclosed in a housing that hydrodynamically couples the transverse motion of the disk to the motion of the thin films of air surrounding the disk. Depending on the clamping ratio, this critical speed is three to ten times higher than the critical speed in the absence of hydrodynamic coupling and does not depend on the strength of the hydrodynamic coupling. Despite the nonlinearity of the underlying model, the critical speed problem is linear and tractable. The linearized free-vibration problem is also computed to verify the stability prediction and to examine linearized damping and stiffness as possible design criteria. The results are relevant to the design of both conventional computer floppy disks and the emerging generation of 100+ MB floppies.