Hey guys! I finally got around purchasing a used GTX 780. It is the Revision A1 model. (manufacturer - Asus). I also got 2 mini 6 pin to 6 pin cables aswell as a 2x SATA to 6 pin, 2x 6 pin to 8 pin and a 6 pin extension cable. I plugged in both of the mini 6 pin cables into the Mac's logic board and connected them to the 2x 6 pin to 8 pin adapter. The 8 pin end went into the graphics card. The 6 pin that came from the 2 SATA connectors I plugged in in tray 3 & 4 got hooked up to the extension cable. The other end of the extension cable runs into the 6 pin connector on the 780. When I power on the machine I get no boot and the screen stays black. When I put back in my Apple Radeon 5770 everything boots up flawlessly. To rule out it's a cable problem I used an EVGA Supernova 650W PSU that I connected to the graphics card. I then did the paper clip trick to power on the PSU. Still, not chime / black screen. The white LED is constantly lit. I do have Mavericks 10.9.5 (13F1077) installed. I installed the latest Quadro / Geforce driver from the nvidia website and selected it in the menu bar. Latest CUDA drivers are also installed. So I guess my card is faulty or it's some software issue. What do you guys think? Could it be the security update 2.0 thingie? IIRC it was responsible for many "dead" cards in Yosemite, however I am on Mavericks. I already tried the card in PCI slot 2 btw. If you guys have any idea I would be very thankful!