Is anyone else having click and drag problems with their Mac Pro INTEL computer? Click and drag seems to only work half the time, the other half it will not select, but open the file instead.
Have you tried a different mouse?
Have you checked the mouse clicking settings in the System Preferences/Keyboard & Mouse pane? (double-click speed)
If these are both good, have you checked for the same problem after logging in as a different user?
