Any updates on this? I'm experiencing the same issue with a mid 2010 Macbook Pro and Mid 2011 Macbook Air (Pro can see the Air, but not vice versa except if the pro is sending a file to the air). I've not gone through the reboot and such to see if that makes it available for a period of time (i just use dropbox as the backup), but i was curious if you were ever able to find a solution.