https://www.youtube.com/watch?v=MflgEGvxRls So, I updated to 10.9.2, and my external monitor (Dell U2713) doesn't work anymore through one of the Thunderbolt ports on my 15" Retina (late 2013). The other port still works fine. It's most definitely the update, as the problem occured during the exact time the update was applied not before, not after. I'm getting kind of vexed by these stupid things (monitor not working, sound disappearing, magsafe not charging every third attempt) after paying $3400 for a computer. Googling doesn't help too much I've seen people with similar issues, but nothing else. I tried an SMC reset already.