If I have my LG 34UM95 Ultrawide display attached with Thunderbolt, when I hold down the option key on restart to choose boot disk, I get blank screen/no signal. If I do the same connected with HDMI instead, I get the screen showing startup choices. Similar problem using boot camp / Windows 8.1. If I go through the process of using HDMI video to boot into Windows, I can use the split screen feature to show both Thunderbolt and HDMI inputs, and sometimes I can see the Thunderbolt screen showing a mirrored desktop, other times it remains dead, black. I've tried unchecking "Fast Start", didn't seem to change anything. I'd like to only use Thunderbolt for video, but I can't boot into Windows without switching to HDMI. I'd be interested in any info or suggestions.