One problem with System 6.0.5 is that, if the connection is somewhat
shaky, the system does not recognize the mouse after startup. If you run
pre-6.0.5 System software, you can start the machine without the mouse,
wait for it to complete the boot process, and then plug the mouse in. The
system will recognize the mouse immediately.
We suggest that you try these diagnostic steps:
1) Verify the mouse connection to the keyboard or back of the machine.
2) Start the machine with the mouse plugged into the connector not
currently in use.
3) If there is no difference, try throwing away the
currently-installed System folder and reinstall a new copy, using the
Installer.