I downloaded the Windows10 October 2018 Update iso file from the Microsoft website Download Windows 10 Disc Image (ISO File). The downloaded file was named Win10_1809Oct_English_x64.iso. I tried using the Boot Camp Assistant on my iMac (21.5-inch, Late 2013) to create a USB flash drive Windows 10 installer. I am currently running High Sierra (macOS 10.13.6). The result was the error message shown below.


The flash drive is 16 GB in size. After some research, I determined the error message is probably referring to the sources/install.wim file on the ISO which is 4.4 GB in size. The file is above the 4 GB limit for files that can be stored on FAT32 formatted volumes. The Boot Camp Assistant formats the flash drive FAT32 and I assume can not copy the sources/install.wim to the flash drive. Thus the displayed error message.

Is there an alternate way way to create the USB flash drive installer using just the software included with macOS? The USB flash drive needs to include the Windows Support software provided by Apple. If so, what is the procedure?

  • You've gotten to the crux of the issue, yes: a FAT32 formatted disk cannot store a file larger than 4 GB's. I resolved this once before, albeit for Window Server. Let me see if I can find those instructions... Dec 17, 2018 at 14:48

4 Answers 4


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.

from hp

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.

Either .esd files are more efficient in storage than .wim files or the install.esd file has less images than the install.wim file.

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.

Answer #1

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.

  1. 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.


  2. 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..


  3. 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.

  4. 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.


  5. 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.

Answer #2

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.

Answer #3

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.

  • For recent Mac users without an optical drive or Windows access: If you are making the installer for a UEFI system, the third suggestion (the April ISO) avoids the filesize limitation of FAT32. (I saw suggestions elsewhere revolving around MBR, but this was the simpliest solution.) Feb 24, 2019 at 4:48

I spent some looking for this answer and eventually found this guide that allowed me to create Windows 10 USB boot from a macOS system. With this you don't need the Windows Support Software.

  1. flash the USB drive to FAT32
  2. mount the ISO and copy over all the files except for the .wim file which is larger than the 4GB size limit.
  3. use a tool called wimlib to split and copy the .wim file to the USB.
  4. Insert your USB which will now contains the full Windows installer.
  • I wonder which versions of Apple and Microsoft software you are using. The problem described in this question has be resolved by using the current versions provided by both companies. Feb 21, 2020 at 4:29
  • I was using MacOS Mojave 10.14.6 and the Windows 10 November 2019 update. I came across this question when trying to create a bootable USB drive for Windows 10 from a Macbook Pro, with no access to a Windows machine. The .wim file in that version is greater than 4GB and therefore cannot be easily transferred or copied to FAT32. Feb 21, 2020 at 18:47
  • The original question has to do with installing Windows 10 on a Mac. I assume you are creating a Windows 10 installation flash drive for use on a machine that is not a Mac. Feb 21, 2020 at 23:42

I had to use Windows 10 1803 (April) NOT 1809 (October)

I tried everything, hours and hours of troubleshooting and live in this kind of tech

  • What is the model/year of your Mac? What version of macOS do you have installed? What is the name of the Windows ISO file? Did you download the file from the internet? If so, from which site? To say you failed does not mean much unless you give enough information to help others. May 17, 2019 at 21:10
  • @DavidAnderson 2015 imac 27" with Mojave (although this issue applies to a broad range of macs), It's the windows iso directly from the MSFT website, specifically for english x64. This was a tough issue because bootcamp is effectively useless unless you go through a superfluous amount of hoops so make it work. Good luck.
    – Jacksonkr
    Dec 21, 2019 at 19:05
  • The 2015 and newer model Macs install Windows through the Boot Camp Assistant by coping the files in the Windows ISO to a temporary partition on the internal drive. This allows installation without the use of a DVD or flash drive. Originally this temporary partition was FAT32 formatted which caused problems when the ISO contained files to large to be stored. However, the Boot Camp Assistant now ExFAT formats the partition which eliminates this problem. Dec 21, 2019 at 20:25
  • I also had a LOT of issues with Apple's virtual partitioning which seemed new as of High Sierra / Mojave so I manually converted partition types back to NTFS / Mac OS Extended. Bootcamp really seemed to struggle with this. Did you find any neat tricks to get around Apple's virtual partitions?
    – Jacksonkr
    Jan 2, 2020 at 22:55
  • By Apple's virtual partitioning, do you mean APFS? By Bootcamp do you means using the Boot Camp Assistant to install Windows 10? Jan 2, 2020 at 23:04

If you need to get the newer versions of Windows 10 into a Fat32 USB drive then another option is to make the ISO have smaller (WIM) files in it by using this tool:


(my motivation was to recover a Windows machine that only supports Fat32)

(the warning is true, if you are looking to do bootcamp on a newer macbook just use exFat)

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