  We study the transition from the geometrically thin disk to the hot coronal
flow for accretion onto black holes. The efficiency of evaporation determines
the truncation of the geometrically thin disk as a function of the black hole
mass and the mass flow rate in the outer disk. The physics of the evaporation
was already described in detail in earlier work (Meyer et al. 2000b). We show
now that the value of the viscosity parameter for the coronal gas has a strong
influence on the evaporation efficiency. For smaller values of the viscosity
evaporation is less efficient. For a given mass flow rate from outside the
geometrically thin disk then extends farther inward. Spectral transitions
between soft and hard states are then expected for different mass flow rates in
the outer disk. The physics is the same for the cases of stellar and
supermassive black holes systems.
