I've had my MacBook Pro since late 2006 running Tiger. I've sent for the Leopard upgrade and it should arrive tomorrow with a bit of luck. Is the Time Machine feature just an automated backup program that runs in the background all the time? If it is...does it slow the system down? Since I have only around 50Gb HD space of my 120Gb HD left I was wondering if Time Machine can be set to use an external HD connected by USB such as my WD MyBook 250Gb?