Some Mac G4 users have reported a problem where attempting to shut down merely restarts their Mac – in other words, the computer won’t shut down. This usually has to do with either a USB device or the Energy Saver preferences. Follow the following steps to remedy the situation:
Step 1: The initial thing to try if you have USB devices connected to your Mac is to power them off and disconnect them. Then shut down the Mac. If this is successful you can reconnect the USB devices, power them on, and restart your Mac.
Step 2: Then, make sure everything’s working OK, and see if the Mac will shut down with the USB devices connected and on again. If it still won’t, trash your Energy Saver Preferences after the next restart (in System Folder: Preferences), open the Energy Saver control panel and set ‘Sleep’ to “Never”, then restart.
Step 3: At this point, assuming everything is working properly when your Mac restarts, you want to start over at Step 1. We’d be surprised if your Mac didn’t shut down by this point, but if the Mac STILL won’t shut down, well, that’s pretty strange – you’ve entered the forbidden realm. You should make sure all of your USB devices have the most current drivers and software installed; if that doesn’t work you may have to turn off your external USB devices first each time before you shut down your Mac.