My Optiplex 990 (with EVGA NVIDIA Gtx650 Ti 1024MB) runs on OS X 10.11 El Capitan. What to do? http://www.tonymacx86.com/el-capita...-el-capitan-any-supported-intel-based-pc.html (For whom might read this in the future. Post-installation with Multibeast 8.0 was not available at that time) I did a clean install with bootable (LEGACY) USB. To boot from usb and enter the installation I had to modify one entry in .../EFI/CLOVER/config.plist: Graphics -> Inject -> Intel -> false (changed it also after installation on the HD) After Installation I could boot El Capitan from the USB Clover and it ran smoothly (except sound), but after the Post-Installation procedure (see link) there were some problems. I tried a lot of things at first that didn't work, so I don't know if I forgot any important thing. But the essential configurations are: For the "Stepper Error 18": turn off "INTEL Speed Step" in your BIOS Install NullCPUPowerManagement.kext, delete AppleIntelCPUPowerManagement.kext in S/L/E If someone has a patched DSDT.aml for BIOS revision A19 (please share!) than those steps might not be needed. I also changed the default SMBIOS from the default iMac 12,2 to MacPro3,1 ... but I don't know if it made a difference. Sound works with of VoodooHDA (I use the current version 2.8.8). The "white noise" problem is (permanently) fixed by copying VoodooHDA.kext from S/L/E on your Desktop, Right Click "Show Package Content", edit .../Contents/Info.plist by changing the integer value of the gain under IOKitPersonalities->VoodooHDA->MixerValues->iGain to 0. Save it and install the edited VoodooHDA.kext with something like Easykext Utility Thanks a lot, enzosdue, for the original post.