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Funny, my girlfriend just asked me no less the 2 minutes ago what this means. Its just updating caches that make the system boot faster ... just wait for it to finish.
 
Hey mate have u tried the volume up button on the keyboard, is it synced with the tone that comes when u press it? Seems like a bug to me...
 
Yeah, it's completely normal. On Tiger this used to occur in the background after installing an update. That resulted in the first boot being slow and sometimes unresponsive. Seems like Leopard has a better implementation by refreshing the caches before the reboot.
 
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