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

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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).

Best Answer

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 !

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