May 31, 2019. Updated to 10.14.5 without any problem. I used the same kexts. However there are two important notes:

1. Update to the latest CLOVER: Clover_v2.4k_r4934
2. Copy the efi drivers at https://github.com/acidanthera/AppleSupportPkg/releases to the directories EFI/CLOVER/Drivers64UEFI and EFI/CLOVER/Drivers64

June 29, 2018: @Andreas Henriksson fixed USB 3.0 problem in a comment here.

Updated my macOS to 10.13.5 using App store without any problem.

Note: I recently updated my PC to 10.13.2, I put my EFI/CLOVER directory for macOS 10.13.2 in 10.13.2 directory. Have fun!

As @Tomygames suggested at here. You need to "[a]ctivate Legacy ACPI CPU Tables in BIOS".

Some detailed instructions to install macOS provided by @spellbound can be found here.

How to install MacOS Sierra 10.12.3 on HP Z420 Workstation.

Overall, we will use Unibeast to create a USB Installer.

From my Mac OS X 10.10 or 10.11, download macOS Sierra Installer (go to App Store to download).

Then, we use the recent Unibeast (UniBeast 7.0.1) to create a bootable USB. However, we have to use kexts from CustoMac Essential package and put these kexts to /Volumes/EFI/EFI/CLOVER/kexts/Other/ directory (mount your EFI partition first!)

Note: I put my /Volumes/EFI/EFI/CLOVER directory and all kexts we need in the repo.

• Install the latest CLOVER (e.g., v2k4)

• We have to use flag npci=0x2000 all the time. (I also use dart=1)

• Z420 crashes randomly when booting, we can fix that with flag cpus=1. However, this makes our PC slow (only 1 CPU!!), therefore, we fix this problem by installing VoodooTSCSync.kext at /System/Library/Extensions . Remember to fix permission. E.g.

  chmod -R 755 /System/Library/Extensions/* && chown -R root:wheel /System/Library/Extensions/* && kextcache -system-caches.

• We got problem with USB ports (therefore, cannot install from USB). We fix it by:

  • First, Use Clover Configurator V4.39.1 app to mount EFI and patch ACPI (Look at "List of Patches" in Acpi menu and apply All Of Them)
  • Install USBInjectAll.kext to inject all USB ports!

• HP Z420 crashes with many versions of FakeSMC and NullCPUPowerManagement.kext, so please use those in the repo!!!

• At first, when we don't have Nvidia WEB drivers, we need to disable NVidia kext by flag nv_disable=1. We later can install latest Nvidia Web driver for 10.12.3 (https://images.nvidia.com/mac/pkg/367/WebDriver-367.15.10.35f01.pkg), and we will not need that flag anymore.

• Use SMBios of Mac Pro 5.1.

• The system cannot wake up after going sleep. We should set Computer Sleep to Never in System Preferences -> Energy Saver.