Short Version of this Guide >> http://fgimian.github.io/blog/2016/03/12/installing-windows-10-on-a-mac-without-bootcamp/

Needed:

MacOS Already Installed and working

USB w/ Windows 10 formatted for EFI

BootCamp Drivers >> https://support.apple.com/downloads/boot-camp

GPTFDisk Binary Package for MacOS >> https://sourceforge.net/projects/gptfdisk/files/gptfdisk/1.0.3/gdisk-binaries/ (Get the one ending in .pkg)

Setup:

Step 1 A & B are done in Recovery Mode

A: Partitioning HD in Recovery  

B: Disable "rootless environment" >> https://superuser.com/questions/508026/windows-detects-gpt-disk-as-mbr-in-efi-boot (See El Capitan Addendum:)

csrutil disable

Step 2: Confirm Protected MBR >> https://superuser.com/questions/508026/windows-detects-gpt-disk-as-mbr-in-efi-boot

Check disk status via

sudo gdisk /dev/disk0

Step 3: Install Windows

Step 4: Bootcamp/Mac Hardware Tweaks

TEST REBOOTING INTO EACH OS

Afterwards you are going to want to setup read/write access to each partition

Share Mac + Windows NTFS/HFS Read/Write Access across both OS:

- Windows is installed on NTFS Drive
- MacOS is installed on HFS+ Formatted Drive

Both will have read access off the hop, but to get write access you will need 3rd party drivers for each OS.

Windows needs HFS+ Drivers to write to Mac OS Partition

MacOS needs NTFS Drivers to write to Windows OS Partition

- Rec is Paragon HFS+ for Windows & Paragon NTFS for Mac
- Free Trial available but costs $

Boot into each OS and confirm you can read/write a simple file to all partitions.