Pumping performance of a disk-type drag pump is studied numerically and experimentally. Molecular transition and slip flows that arise in a spiral channel on the rotating disk are simulated by using molecular and continuum methods. The molecular approach employs the direct simulation Monte Carlo (DSMC) method, and the continuum approach solves the Navier-Stokes (NS) equations. The formulation of second-order slip boundary conditions is utilized in the NS method. Particular attention is paid to matching the DSMC method with the solutions obtained by the NS method in the slip flow regime. In the experimental study, the inlet pressures are measured for various outlet pressures of a test pump. Comparison between the experimental data and the numerical results reveals that the DSMC method provides the more accurate solution of the rarefied channel flow for the range of Knudsen number Kn>0.02 than does the NS method.