Before fully answering the question, a better explanation of the problem needs to be made. Microsoft provides this website to download Windows 10. Below is a snapshot of what Windows users will see. Mac users will not see what is shown below.
Instead, Mac users are automatically redirect to this website. Below is a snap shot of the website.
When using the Windows operating system, you can download the application
MediaCreationTool1809.exe. This allows the creation of either a USB flash drive installer or the download of an ISO file. Mac users do not download an application. Instead, Mac users can only download the ISO file. According to Apple, the Boot Camp Assistant application is suppose download the Window Support software and read the ISO file to create the USB flash drive installer. Currently, the Boot Camp Assistant fails to do this.
Note: The MediaCreationTool1809 creates a FAT32 formatted USB installer. The Boot Camp Assistant is suppose to also create a FAT32 formatted USB installer, but fails.
So there must be a difference between the files downloaded for the USB installer and the files downloaded in the ISO.
I do not know how many of the files are different. However, I do know the largest file is different. This file contains all the Window 10 images. The MediaCreationTool1809 application creates the file
install.esd which is 3.45 GB in size. This is nowhere near the 4 GB limit for files stored on FAT32 formatted volumes. Unfortunately, the ISO substitutes the file
install.wim which is 4.4 GB in size. This is well over the 4 GB limit.
.esd files are more efficient in storage than
.wim files or the
install.esd file has less images than the
So, this explains what is wrong. The Boot Camp Assistant can not transfer the
install.wim file from the ISO to the FAT32 formatted USB flash drive. Next, I will post three answers. I am sure there are many other possible answers.
Mac computers running High Sierra or Mojave can boot from USB flash drives that are ExFAT formatted. So, Apple could fix this problem by changing to Boot Camp Assistant to format the USB flash drive as ExFAT instead of FAT32. For now, you can use the following steps to create a USB Windows 10 flash drive installer.
Use the Boot Camp Assistant application to download the Windows Support Software. Look for
Download the Windows Support Software under
Action on the Menu Bar. After the download finishes, the Window Support Software folder should appear as shown below.
Use the Disk Utility application to erase a 16 GB or larger USB flash drive. Choose Name, Format and Scheme as shown in the image below..
Use the Finder application to mount the ISO file. The name of the ISO file, I downloaded, was
Win10_1809Oct_English_x64.iso. The volume name, when mounted, was
CCCOMA_X64FRE_EN-US_DV9. Your names may be different.
Use the Finder application to copy the contents of the ISO file to the USB flash drive. When finished, the root folder of the USB flash drive should appear as shown below.
Use the Finder application to copy the Window Support Software to the USB flash drive. When finished, the root folder of the USB flash drive should appear as shown below.
The USB flash drive installer can now be used to install Windows on your Mac. If wish to use the flash drive to install Windows 10 on a different Mac, then you should replace the Windows Support Software files with ones downloaded on that Mac.
This answer was suggested (in a now deleted comment) by Solar Mike. I will remove the answer, if he posts a similar answer. Technically, the answer does not fall within the guidelines of the question, but is worth consideration.
Use a machine running Windows to create the USB flash drive installer. At this point the flash drive does not contain the Windows Support Software provided by Apple. To add this software, follow steps 1 and 5 of Answer #1.
Download the Windows 10 May 2019 Update ISO file from the Microsoft website Download Windows 10 Disc Image (ISO File). This ISO file is compatible with the Boot Camp Assistant.
Note: This answer #3 was tested using the English 64 bit Windows 10 May 2019 Update ISO file. This answer may not work with other versions of the Windows 10 May 2019 Update available for download.