  The possibility that some of the observed X-ray and optical variability in
active galactic nuclei and galactic black hole candidates is produced in
accretion disks through the development of a self-organized critical state is
reconsidered. New simulations, including more complete calculations of
relativistic effects, do show that this model can produce light-curves and
power-spectra for the variability which agree with the range observed in
optical and X-ray studies of AGN and X-ray binaries. However, the universality
of complete self-organized criticality is not quite achieved. This is mainly
because the character of the variations depend quite substantially on the
extent of the unstable disk region. If it extends close to the innermost stable
orbit then a physical scale is introduced and the scale-free character of
self-organized criticality is vitiated. Significant dependence of the power
spectrum density slope on the type of diffusion within the disk and a weaker
dependence on the amount of differential rotation are noted. When general
relativistic effects are incorporated in the models, additional substantial
differences are produced if the disk is viewed from directions far from the
accretion disk axis.
