  By studying three-dimensional, radiative, global simulations of
sub-Eddington, geometrically thin black hole accretion flows we show that thin
disks which are dominated by magnetic pressure are stable against thermal
instability. Such disks are thicker than predicted by the standard model and
show significant amount of dissipation inside the marginally stable orbit.
Radiation released in this region, however, does not escape to infinity but is
advected into the black hole. We find that the resulting accretion efficiency
($5.5\pm0.5\%$ for the simulated $0.8\dot M_{\rm Edd}$ disk) is very close to
the predicted by the standard model ($5.7\%$).
