Late last year I -posted a help query when a Intel Macpro would not shutdown, restart or log out from the Apple menu. All short term solutions failed to solve the problem.
Apple reached a dead end also. Only answer was to erase.
After hoping for a better solution, nothing better eventuated. I erased HD and recovered data files.
All is OK again.
The only cause I can think was an attempt to copy a DVD using Toast. Maybe copy protected. If this is so, beware.