UPDATE 2: Just tried deleting all the DisplayVendorID entries in /System/Library/Displays/Overrides, also to no avail.
Also tried deleting /Library/Preferences/ to no avail. UPDATE: Tried 10.9.3 beta 3 and I'm having no luck getting my second monitor working, though the 4K display is still working fine. Gonna test on beta 3 to see if I run into the same problems as you guys. One problem I ran into with beta 2 was that my card would only drive my 4K monitor, and not my 27" DVI display. Download ASUS VivoBook Flip TP501UQK Realtek LAN Driver 10.9.422.2016 for Windows 10 64-bit (Network Card). Maybe you'll find my plist configs useful. In this article, we’ll explain the necessary fix process for each OS version. I'm using Chameleon v2.2svn r2377 right now. Apple’s recent security update, 2015-002, disabled Nvidia’s Web Driver, much to the dismay of many OS X 10.8, 10.9 & 10.10 users. They're not in 10.9.2 or the 331.01.01 web drivers, and I cannot comment on 10.9.3 sorry. I did try the NVIDIA web drivers when I was trying to get 60Hz working initially, but I'm 95% sure I wiped all traces of them from my system. I don't think that drivers for the Maxwell based 750 cards have got into an OS X or web driver release yet - The card's just too new.
REMARQUE : Si, pour toute raison, vous ne parvenez plus à accéder à votre bureau système et souhaitez restaurer le pilote original Mac OS X version 10.9.5 (13F34), vous devez réinitialiser la NVRAM de votre Mac.
The (clone of the) 10.9.2 install I upgraded to 10.9.3 booted just fine before the update, and I'm writing this from another 10.9.2 install without booting problems. Une fois le pilote NVIDIA Web et le gestionnaire de pilotes NVIDIA désinstallés, cliquez sur Redémarrer. Out of interest, did you have the NVidia web drivers installed before updating to 10.9.3 beta? What bootloader do you use?