There’s nothing in the UEFI specifications that prevents booting from an NTFS formatted USB flash drive. In fact, this so-called limitation is entirely artificial. Luckily, most modern computers can boot from a single-partition NTFS formatted USB flash drive. Thus, one can indeed install Windows 10 from a custom WIM image larger than 4 GB, which is the the maximum file size on FAT32 media.
But, unfortunately, there are still a number of computers where BIOS / UEFI lacks proper drivers. That’s the real reason why they can only boot from FAT32. I have an Asus laptop: it’s only a few years old, and it simply refuses to boot from NTFS. Deploying my custom Windows image on that laptop from USB, I have been forced to split the 12 to 20 GB custom install.wim files that I create, so I can deploy them using DISM instead of running Windows Setup. For those interested in how to do this very thing, I’ve written a tutorial on TenForums.com.
Early this morning I had to make Windows 10 version 1803 multi edition USB install media. I had the Consumer Editions ISO, downloaded from my Visual Studio subscription, and I wanted to use this specific ISO. For this multi edition ISO, the install.wim file is annoyingly 0.3 GB too big to fit in FAT32 formatted USB flash drive:
OK, no problems, I thought, thinking that I would just split the WIM using DISM. However, knowing my friend for whom I was making this installer USB is not familiar with manually deploying from a split WIM, it would be easier for me to make the ISO bootable from a FAT32 partition. Then, he could run Windows Setup from an NTFS partition. Knowing his hardware, I suspected that at least on two computers he wants to reinstall he would not be able to boot from NTFS. So, I had to create FAT32 media to do the job instead.
The process is remarkably easy. Here is how I created Windows 10 USB install media which boots from a small FAT32 partition, then gives control to a larger NTFS partition which contains the 4GB+ install.wim file.
The first step was to remove the existing single FAT32 partition that used the full 8 GB capacity of the USB. Next, I created a 1 GB FAT32 partition and used the rest of USB’s capacity for an NTFS partition. I mounted the ISO, selected everything except the Sources folder, and copied these files and folders to the small FAT32 partition:
Now I created an empty folder on this FAT32 partition, labelling it Sources, and copied Sources\boot.wim file from mounted ISO to this folder:
Then I simply copied all of the ISO’s content, including the complete Sources folder to the bigger NTFS partition on the USB drive. Now I had a USB flash drive to install Windows 10 which boots from a small FAT32 partition, then runs Windows Setup from the bigger NTFS partition:
The difference between these two partitions is that on FAT32 partition, the Sources folder only contains one single file, the boot.wim file that’s required for the USB to be bootable.
That’s it. This USB flash drive can be used to boot any BIOS / MBR and UEFI / GPT computer. Because Windows Setup is run from NTFS partition, you can use this method for your custom install media with larger than 4 GB install.wim files.
Author: Kari Finn
A former Windows Insider MVP, Kari started in computing in the mid 80’s writing code for VAX / VMS systems. Since then, he’s worked in a variety of IT positions. He specializes in Windows image capture, customization, repair and deployment as well as Hyper-V virtualization. Kari is a proud Team Member at number #1 Windows site TenForums.com.