These are the steps I use to create a Window 10 installer on MacOS. I only do this once in a blue moon and I want to make sure I keep track of these commands before they are lost in my history.

# Format the USB drive as MS-DOS and name it "WIN10"
$ diskutil eraseDisk MS-DOS "WIN10" MBR /dev/disk127

# Copy files from mounted Windows 10 ISO to new drive
#   > Make sure to preserve trailing slash (/)
$ rsync -vha --exclude=sources/install.wim /Volumes/CCCOMA_X64FRE_EN-US_DV9/ /Volumes/WIN10

# Split and copy the last file
$ wimlib-imagex split /Volumes/CCCOMA_X64FRE_EN-US_DV9/sources/install.wim /Volumes/WIN10/sources/install.wim 4000

The prerequisites for the above commands are knowing the id of the mounted USB drive and wimlib-imagex. MacOS only has write capabilities for the older MS-DOS file format which has a 4GB single file size limit. This is why we use wimlib-imagex to split the 5GB install.wim into two files.

You can find the ID of your USB drive with Disk Utility.

It looks like I installed wimlib-imagex with HomeBrew.

$ brew install wimlib

See you later future Baylor!