- Access Recovery via pressing Option + R when powering on Mac
- Open Disk Utility
- View All Devices
- Create New Partition by hitting + symbol in blue
- You must change the partition size before selecting to FORMAT as MS-DOS(FAT)
- We will add a 3rd Partition later
B: Disable "rootless environment" >> https://superuser.com/questions/508026/windows-detects-gpt-disk-as-mbr-in-efi-boot (See El Capitan Addendum:)
- Reboot and unplug everything but USB
- Load USB
- NOTE IF YOU DON'T SEE "EFI Boot" your USB isn't formatted correctly!
- When selecting drive, delete previously created partition and create new NTFS PartitionThis is a good time to create the 3rd "Shared" Partition
- Note My windows rebooted more than normal, but it finally got to the "Setup" screen
Boot into each OS and confirm you can read/write a simple file to all 3 partitions.