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Creating Windows 10 Installation Media from a Mac

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Most computers can only boot from USB sticks formatted as FAT32, which has a 4GB file size limit. The latest Windows 10 installation media includes a install.wim file larger than 4GB, which cannot be directly copied to a FAT32 USB stick.

There were a number of solutions to this listed online:

  • Splitting install.wim into 2 files
  • Optimizing install.wim so it was smaller than 4GB
  • Converting the Windows 10 ISO into a RAW image, which is copied directly onto the USB stick via dd
  • Using two USB sticks

The first 3 solutions did not work for me. Whatever I did I ended up seeing one of these errors:

  • "Windows could not prepare the computer to boot into the next phase of installation"
  • "A media driver your computer needs is missing"

The last solution, using two USB sticks, did work!

The dual USB stick method

I can no longer find the forum post describing this method, so allow me to redocument it.

The Windows setup can be booted without a complete set of installation files, and when it gets around to installing windows it appears to search all connected drives for the relevant files.

We can take advantage of this by creating one USB stick as FAT32 that will boot the Windows setup, and another supplementary USB stick as ExFAT with all of the required installation files.

First of all, download and mount the Windows 10 Disc Image (ISO file).

Creating the Boot USB

This USB stick will contain only the files necessary to boot the Windows setup process.

  • Format the USB stick as "MS-DOS (FAT32)"
  • Copy across all of the ISO file's contents, except for the "sources" directory
  • Create your own sources directory on the USB stick
  • Copy across sources/boot.wim from the ISO to the USB stick

Creating the supplementary USB

This USB stick will contain all of the installation files for the separately-booted Windows setup to use.

  • Format the USB stick as "ExFAT"
  • Copy across all of the ISO file's contents

Then plug both of these into your computer, and you should be able to install Windows 10 just fine 🙂