With the release of OS X 10.10.5, and as previously mentioned on the blog, we don’t recommend updating your OS X version immediately if you are using an Nvidia GTX 9XX series GPU, with or without a Mac EFI. If you do have an EFI card, we are currently selling the GTX 980 4GB & GTX 980 Ti 6GB with Mac EFI, then you’re safer as you’ll still be able to boot, you just wont get GPU acceleration.
If you have a card without a Mac EFI and perform the update to 10.10.5, you’ll find that you can no longer boot into your system, unless you have an EFI card such as a GT120 lying around. In this case you need to remove your non EFI card, install the EFI card, boot into 10.10.5 and then download the latest Nvidia Web Drivers for Mac (there is a link below).
Either way, you will need a GPU with a Mac EFI installed (and you will need to remove all non EFI cards), when you update to 10.10.5, following this you will need to download and install the latest Nvidia Web Drivers, link below:
Please keep in mind that for future OS X updates, we strongly advise that you wait for the Nvidia Web Drivers to be released, and have a Mac EFI GPU handy, before updating.