I'm setting up an older MacBook Pro (it still has the button on the trackpad) and after upgrading it from a fresh install of Snow Leopard to Lion last night my external mouse is not functioning properly.
Left clicks with the mouse are not registered. Right clicking is fine. Moving the mouse around is fine. But left clicks do nothing. I can click with the trackpad button (but this button sticks, which is why an external mouse is required) and that does work.
I have natural scrolling turned off. I have all trackpad gestures turned off. There are no open windows or any dialogs that might be interrupting things. There are no stuck keyboard keys.
One bit of odd is: if I reboot the machine, for a few seconds after a fresh reboot the left click on the external mouse works fines. And then...nothing. Disconnecting and reconnecting the mouse does not fix the problem, even temporarily.
I've tried multiple external mice, wired and wireless, now and it's the same for all of them. All mice are known to be good and function well on my mid-2007 iMac that is also running Lion.
The only thing possibly related in the Console is the repeated message:
IOSurface: Buffer allocation size is zero
And, it appears after one more reboot, things are working okay for me now. I've been using it for ten minutes without the left button on the mouse failing.