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 \pm 0.5 \% for the simulated 0.8 \dot { M } _ { Edd } disk ) is very close to the predicted by the standard model ( 5.7 \% ) .