I have been using the Sierra beta and for the last couple months my Thunderbolt Display has been having issues. It turns off randomly and required a restart. Eventually it stopped working at all. Did all the normal stuff, power cycle, reset, SMC reset, etc. No fix. Both a 2013 iMac and 2011 MBA with the same issue. Took Display into Apple and they replaced the power node, thunderbolt cable, and the motherboard. Cost me a pretty penny. Get home and the Display doesn't work at all. Been messing with it a few days and nothing. Today, I sent my MBA back to El Capitan due to a school program I needed to run. On a whim, I plugged in my Thunderbolt Display and viola!! It works. Problem is, it won't work on my iMac with Sierra. I plan to go into Apple Store tomorrow for another reason, but wanted to bring this up here. Anyone else have issues with TB a Display in Sierra? I'm pretty pissed I dropped over $300 fixing a 5 year old Display when it seems to be a software issue.