Steam "not responding" on iMac 5k I've just installed the Steam installer on my new iMac 5k, and downloaded the latest version of Steam onto it. When loading Steam, the login window loads, but all of a sudden the application freezes and needs to be "Force Quit". Any thoughts what a solution to this might be?
Try disabling firewall in Security &amp; Privacy settings, try adding rule to accept all internet traffic for desired application in Firewall Options. 

When I start steam the first time of the day it crashes due error you describe, when I start steam 2nd time everything works just fine.