Hi everybody... I just got a brand new 21.5" iMac, and I connected two external displays through Mini DVI --> HDMI cables. These two displays are each connected to the two Thunderbolt ports on the iMac. When I started the computer for the first time and set everything up, all three monitors (including the iMac's internal one) worked fine, but when I upgraded to Yosemite, I can only get it to recognize one external display at a time (whichever was plugged in first). Anyone have experience with this? I'm making a mountain lion USB as we speak to downgrade... It's a shame! I've tried restarting, resetting SMC, and resetting the PRAM.