I am about to buy a USB external drive to use as a backup drive for my minimac. It looks like Time Machine is the way forward but I have a few questions. From what I can make out after the initial backup you can set Time Machine to backup hourly, weekly, etc. I cannot find out if you have to set specific time or hows it works. Having done the initial backup - if I switch on the next day and download a few photos / surf the net for say 10 minutes and then switch off my minimac does Time Machine kick in and backup? Does Time Machine 'know' the mac has been switched on and so do a backup? I am worried that as I use the mac for sort periods, maybe every other day, Time Machine is not actually going to kick in and do a backup.