I have had multiple customer complaints about this now. A couple either have or are sending back for refund replacement. Sadly, there is nothing we can do. The issue is with 10.9.3, with either version of drivers. U[pper DVI port goes dead. Try 10.8.5, all is good again. I have looked at monitor definition tables, all good. Same as 2GB version of same card. Even more confusing, our GTX770 4GB EFI has all 4 ports working. Same card with higher clocks and different Device id. Only changes. So, there is some buggered up nonsense in drivers which tracks to device id and RAM quantity. I can tell you this, the primary difference between OSX and Windows roms and drivers at this point is how monitors are defined. If I didn't know better I would guess that Nvidia is writing drivers on Hackintoshes instead of real Macs. An EFI card requires different monitor definitions than a PC BIOS card. With each driver iteration there have been these same errors. The GT640 EFI cards are crippled down to 1 display.