My mac goes to sleep (goes black) and then my other pc's/laptops can't see it etc. Is there a way to not turn it off. I want specifically to keep it on so my LIve book duo can be accessed to be able to copy files to it. it seems like once it goes dark I can't get to it (this also occurs when my mac air goes to sleep, my mack can't get to it). There must be a setting somewhere. Thanks.