A million years later... flashing my Hynix with your rom (and flashed it with at least 37 other roms) and nothing succeeds. The thing is that as soon as I get the boot screen and the card is correctly recognized by OS X, but Yosemite is laggy (like when you use a 7300GT on a Mac Pro 1,1).. and I actually hear the fan always at high (not full) speed. I've read maybe 80% of this thread, tried many things, but I can't get Yosemite smooth.. EDIT : it seems I have an old version of the 4870 and it doesn't have OpenCL or something like that. So Yosemite wouldn't work properly. I need to get rid of it and ONLY buy the same design of heatsink..