How to create a Windows 10 bootable USB on a Mac for PC without Bootcamp, when install.wim is too large

My Boot Camp assistant seems to be having problems (first I get not enough space, 40 GB, etc.) which I fixed by making a partition using Disk Utility, but then I didn't get an option that others seem to get to Create a usb bootable drive for windows 7 or later. It right away asked for the ISO and started downloading Windows support software, took forever, and then said there wasn't enough space on the partition (there was).

Without Boot Camp, I tried using dd and that didn't work and just reformatting the USB drive to exFAT (FAT32 didn't seem to work, because the October 2018 update has a file over 4 GB, meanwhile I'm downloading April 2018). What happens is that once I boot up the PC, change the boot order and then select the drive, it just goes back to some VMware kernel OS that was still on it when I got the PC. I've tried running in legacy mode, secure boot is disabled.

Does anyone have any alternatives I could use?

Also I tried plugging the usb (formatted exFAT in to my mac and it seems to recognize it in the boot menu, so I'm starting to think it's something with my motherboard/bios settings).

The open-source package called wimlib, can also "optimize" a wim file. I managed to reduce the size bellow 4Go, my FAT32 usb thumb accepted it, and booted with success

Install wimlib via brew brew install wimlib

Copie the bigfile from the iso volume to your HD Disk (Download example by example) cp /Volumes/CCC.../sources/install.wim install.wim

Compress the file wimlib-imagex optimize install.wim --solid

"install.wim" original size: 4463411 KiB 
Using LZMS compression with 8 threads 
Archiving file data: 9 GiB of 9 GiB (100%) done 
"install.wim" optimized size: 3311533 KiB 
Space saved: 1151878 KiB

You may need sudo, because you could encounter an error like this:

[ERROR] Can't modify "/home/roger/win/sources/install.wim": Permission denied 
ERROR: Exiting with error code 71: 
The WIM is read-only (file permissions, header flag, or split WIM).

Copy the reduced file to your usb drive cp install.wim /Volumes/MYUSB/sources/install.wim

Boot up !

There is an open-source package called wimlib that can create, extract, and modify Windows Imaging (WIM) archives. You can install this library on macOS via Homebrew:

brew install wimlib

After that, you can use following wimlib command to split the wim file:

wimlib-imagex split /win10-iso-path/sources/install.wim /Volumes/MYUSB/sources/install.swm 3000

The 3000 means the file should be split into 3000 MB-sized chunks so it can sit on FAT32 partition.

This should work most of time. However, I heard the usb might not be bootable after copying. Hence, you can directly create Windows 10 bootable USB on Mac (Catalina example)by burning the ISO image to USB and there is no need to extract the ISO. However, you should format the USB to exFAT instead.

So, I needed to use FAT32 for my BIOS to accept it, but both the April 2018 and October 2018 updates had an install.wim file that was over 4 GB (I downloaded the English international version for both, maybe other versions are different?). So I needed to split the file. Here is how I did it for anyone else who has this problem.

I found this amazing guide for it (https://www.tenforums.com/tutorials/103340-dism-split-install-wim-file.html), but it was for Windows so I had to adapt it. I used split terminal command to split the install.wim file.

split -b 3700m /Volumes/CCCOMA_X64FRE_EN-GB_DV9/sources/install.wim install.wim

This creates install.wimaa and install.wimab. I then moved all the ISO files to the FAT32 USB, but with the new install.wim files. This time, when I booted from the USB, I get the Windows boot media, but it obviously cannot find install.wim. So I go to the command prompt and follow the guide and type out the diskpart commands it says (I followed MBR).

After that, I copied the contents of the USB drive to the Windows 10 volume that I created from the guide (I think I used xcopy, but there's other commands for it). Navigated to that volume, went to the sources folder and typed the commands:

type install.wimaa install.wimab > install.wim 

(Once that finished, I deleted the old wimaa and wimab files)

del install.wimaa
del install.wimab

After this I followed the rest of the guide, applied the image, add boot records (The disk volumes for me were different than what the guide should have given me, but that might have been a mistake on my part) and booted my pc back up after removing the USB. If it says operating system not found, this worked for me: https://www.wintips.org/fix-operating-system-was-n0t-found-error-on-windows-10-8-solved/.

This was so complicated for some reason and took me many hours to figure out.

I found the only way for me to make it work was installing windows on VirtualBox, and then use Rufus in the virtual machine to create the bootable usb drive. A bit tedious but it works.