Hi all,
I was having the same problem with Yosemite and thunderbolt to HDMI but have found a sort of a solution or at least a bit more info that might lead to someone having a eureka moment. I have a Mac Book Pro, my partner a Mac Book Air. I bought a thunderbolt to HDMI cable from eBay so my partner could plug her Air into the TV. When the cable arrived, it would not detect our LG TV from either computer. So a bit of playing around and this is what I found.
- I plugged the cable from my Pro direct to the HDMI port on the TV and nothing.
- I also tried from the Pro to the HDMI port on amplifier of the surround sound system and again nothing.
- However, if I plug the cable from my PRO directly to the TV HDMI and select that HDMI port on the TV initially still nothing. But if I then plug a normal hdmi cable from my amp to the Mac Pro's HDMI port, suddenly the thunderbolt cable works and the Display Preferences see the LGTV. I checked that the correct hdmi port was selected for the direct connection and I could even turn off my amplifier to make sure that I was not seeing the wrong signal and it still worked.
It seems that there needs to be either a base signal or an earthing through the MacBook Pro's HDMI port for it to switch into an external monitor mode that will detect a thunderbolt to HDMI connection. This is fine for my Mac but as my partner's Air only has a thunderbolt port and no HDMI port it still doesn't work with her MAC.
Hopefully someone will see this and have a brainwave.
