Whatever operating system you are using to create the USB, you will need to have a Windows 10 ISO, either from Microsoft or your system manufacturer and have a USB drive 8GB or larger (or one with at least 5GB of free space and using the FAT32 filesystem, but using a fresh and empty one is best).
TL;DR
#macOS/Linux
# First try the `bootiso` program, it has options for splitting the WIM for you!
# https://jsamr.github.io/bootiso/
# You need to already have 7zip aka `p7zip` on macOS and Linux, and `wimlib` macOS via `brew` or `wimtools` on Linux
### !!!!!!!!!!!!!!!!!!! ###
#!
#! THE DRIVE PREP STEPS ARE DESTRUCTIVE, THEY WILL WIPE YOUR USB DRIVE, be CONFIDENT IT IS THE RIGHT ONE!!!
#!
### !!!!!!!!!!!!!!!!!!! ###
## macOS USB drive prep
# if [ "$(uname -o)" == "Darwin"]; then
diskutil list # or fdisk -l
##!!!! WARNING, THIS STEP WIPES THE DRIVE!
diskutil eraseDisk MS-DOS "WIN10" MBR /dev/disk2 # use the drive that is the USB you inserted and want to use
# The drive should get "mounted" automatically so we can use it to set the target drive
export USB_DRIVE=/Volumes/WIN10
## End macOS USB drive prep
## Linux USB drive prep
# if [ "$(uname -o)" == "Linux" ]; then
lsblk # this should show all drives and indicate the removable ones
fdisk -l
##!!!! WARNING, THESE fdisk AND mkfs STEPS WIPE THE DRIVE!
sudo fdisk /dev/sdX # use the letter of the USB you inserted and want to use
fdisk> o # gives you a clean partition table of type MBR see https://wiki.archlinux.org/title/Fdisk#Create_new_table
fdisk> n # create new partition
fdisk> p # primary partition type
fdisk> <enter> # start at beginning of disk
fdisk> <enter> # use whole disk
fdisk> l # prints filesystem type list
fdisk> t # change filesystem type
fdisk> b # select option for FAT32
fdisk> a # set partition to "active" so it is bootable
fdisk> w # write the changes
fdisk> q # quit fdisk
mkfs.fat -F 32 /dev/sdX1 # use the same letter and we are specifying the first partition we just created
# You might not need sudo, but it avoids permissions issues
sudo mkdir /media/WIN10
sudo mkdir mount /dev/sdX1 /media/WIN10
export USB_DRIVE=/media/WIN10
## End Linux USB drive prep
# Common steps macOS/Linux
export ISO_FILE="$HOME/Downloads/Win10_*"
set -a; : ${TEMP:=/tmp}; set +a;
## if you might want to create multiple installer USB drives without performing the extractions and splits each time, you should copy the first set of files from the ISO as well and then just copy the contents of eg "$TEMP/usb/" to the drive with `rsync` for easy resuming if something happens
# mkdir -p "$TEMP/usb"; 7z x '-xr!install.wim' "$ISO_FILE" -o"$TEMP/usb/"
7z x '-xr!install.wim' "$ISO_FILE" -o"$USB_DRIVE"
7z x '-ir!install.wim' "$ISO_FILE" -o"$TEMP"
wimsplit $TEMP/sources/install.wim $USB_DRIVE/sources/install.swm 3000
## The wimsplit might run faster if you have the free space space available to split it within $TEMP and then copy the separate swm files over as that should result in a continuous write instead of thrashing the USB with a bunch of small writes, doing this before the other files might also result in a bit of a speed-up as flash drives tend to slow down as they get fuller due to the wear leveling trying to find unused blocks or having to clear used blocks (they don't have TRIM like SSDs)
# wimsplit $TEMP/sources/install.wim $TEMP/install.swm 3000; cp -f $TEMP/install*.swm $USB_DRIVE/sources/
## If you are making multiple drives
# wimsplit $TEMP/sources/install.wim $TEMP/usb/install.swm 3000; rsync -aVP $TEMP/usb/ $USB_DRIVE # untested
cd $USB_DRIVE
sync # this requests the OS flush any pending writes to disk, it can take a while or return immediately depending on your system
cd $HOME
# macOS
diskutil eject /dev/disk2
# Linux
eject /dev/disk2 # might need sudo?
It isn't a huge speed increase, but if you were writing several USBs it would add up significantly
$ time wimsplit $TEMP/sources/install.wim $USB_DRIVE/sources/install.swm 3000
Splitting WIM: 5057 MiB of 5057 MiB (100%) written, part 2 of 2
Finished splitting "/tmp/sources/install.wim"
real 20m43.110s
user 0m43.198s
sys 0m6.442s
$ time { wimsplit $TEMP/sources/install.wim $TEMP/install.swm 3000; cp -f $TEMP/install*.swm $USB_DRIVE/sources/; }
Splitting WIM: 5057 MiB of 5057 MiB (100%) written, part 2 of 2
Finished splitting "/tmp/sources/install.wim"
real 18m45.550s
user 0m41.271s
sys 0m10.547s
You probably stumbled across this post because you got an error trying to use Unetbootin or 7zip or hdiutil and diskutil or Disk Utility when trying to make a bootable Windows USB. This might have been E_FAIL from p7zip/7zip, or a hard to spot error in the logs when you cp or 7z x the entire contents of the ISO over to a FAT32 USB. Since FAT32 can only handle files up to 4GB (it truncates anything larger), the "fluffy" install.wim that exceeds this limit gets corrupted and results in a USB that you can boot from, but fails partway through attempting to install Windows, luckily it usually stops before it deletes the partitions that currently exist on the destination hard drive, so you may still have a bootable system that you can follow these directions to salvage or properly prepare the USB drive.
- You have an ISO downloaded to
$HOME/Downloads/Win10_1903_V2_English_x64.isoor your native language equivalent. - You have a USB drive you have formatted with FAT32, either using "Files" in ChromeOS or Disk Utility or Gparted or the command line equivalents (fdisk, mkfs.fat32, and/or parted) in Linux or the Disk Utility or the command line
diskutilin macOS or just formatting the drive by right clicking and doing "Format" in Windows... but why aren't you using the Microsoft tool if you have Windows available (maybe you like doing it the "hard" way to learn more about how things work?). - You have a little patience, internet available and have the right permissions to install a couple packages required to achieve our goal of a bootable Windows 10 USB.
If you have a drive that boots but gives an error message, you can skip a few of the steps below and save yourself a lot of time. The main bits you'll care about are installing the required packages below, getting setup (aka having the ISO and USB drive mounted at known paths), and then you should delete the sources/install.wim from your USB, and then skip to the extricating portion below.
For ChromeOS (with Linux apps), or Linux or macOS we will need the wimtools package and the 7zip CLI aka p7zip package. You can sudo apt update && sudo apt install wimtools p7zip in your Linux terminal or on macOS once you have Homebrew from https://brew.sh you can run brew install wimlib p7zip. On Windows we only need 7zip, you can install that by downloading from their site, or better yet get the fantastic Chocolatey tool from https://chocolatey.org/install and it includes a basic version of 7zip as part of its install, so we don't have to find and install the GUI version.
For Linux and macOS (and on Windows 8.x+) we could technically "mount" the ISO to make the files available instead of extracting them to the USB using 7zip, but I wanted to make the instructions consistent across the platforms as well as easier to copy and paste for fewer mistakes.
Once you have plugged in and formatted your USB drive to FAT32 (use a name like Win10-1903 to make it easier to identify), then make note of the path where it is "mounted".
- On ChromeOS you will need to right click the drive in the "Files" app and select "Share with Linux", this will create a link at
/mnt/chromeos/removable/<drivename>, eg/mnt/chromeos/removable/Win10-1903. You should right click the Downloads folder and "Share with Linux" as well so we can access the ISO file. - On Linux depending on your system it may be at
/media/Win10-1903or$HOME/.gvfs/Win10-1903or/media/$USER/Win10-1903. - On macOS the USB will be under a path like
/Volumes/Win10-1903. - On Windows the USB drive will have a letter viewable in Windows Explorer or in Powershell with
Get-PSDriveor something like that.
If you are lazy like me, you don't want to type out the path several times, so you can use export ISO_FILE=/path/to/your/Win10.iso ie export ISO_FILE=$HOME/Downloads/Win10_SOME_VERSION.iso or on Windows in Powershell $env:ISO_FILE=C:\Users\YourUserName\Downloads\Win10.iso.
Then you can do the same for the destination drive, export USB_DRIVE=/path/to/your/USB_mount or $env:USB_DRIVE=D:\.
We need to do this work in a couple stages, mainly because there is a "plump" install.wim file that is ~4.3GB which isn't compatible with the FAT32 file system which is limited to 4GB files.
The x argument to 7zip stands for eXtract, the -x[r]!wildcard tells it to exclude recursively any file named install.wim (if we don't tell it recursive, we have to specify the precise sources/install.wim and if it ever moved the command would fail). The -i[r]!wildcard means include recursively any file named install.wim which also ends up resolving the whole path of the file sources/install.wim like the exclude above.
First we'll extract everything BUT that file to the USB drive.
7z x '-xr!install.wim' "$ISO_FILE" -o"$USB_DRIVE"
If you are updating an existing drive to the same version, you may want to use -aos to skip existing files, or -aoa to overwrite the files without being prompted.
7z x '-xr!install.wim' "$ISO_FILE" -o"$USB_DRIVE" -aoa
or for Windows since we are doing this a couple times, let's save our fingers.
Set-Alias 7z C:\ProgramData\chocolatey\tools\7z
So now we can just 7z x '-xr!install.wim' "$env:ISO_FILE" -o"$env:USB_DRIVE"
This can take quite a while, take a bio break or grab a soda from the fridge.
In Bash or other Linux shells this should set TEMP if it isn't already.
set -a; : ${TEMP:=/tmp}; set +a or export TEMP=/tmp if the next command throws an error
We need to extract the "voluptuous" install.wim to a temporary location so we can give it a makeover (the lack of a space between -o and "$TEMP" is actually REQUIRED by some weirdness in 7zip).
7z x '-ir!install.wim' "$ISO_FILE" -o"$TEMP"
or for Windows (assuming you used the Set-Alias above
7z x '-ir!install.wim' "$env:ISO_FILE" -o "$env:TEMP"
Now we need to split the file into smaller pieces so it fits within the constraints of FAT32, sadly Spanx don't quite cut it here. The final argument (the number) is in megabytes, the lowest you might want to go is 100, but anywhere between 100-4000 should work, I use 2000 or 3000 to give FAT32 some breathing room, sadly the larger you go the longer it takes before it prints any progress (on non-Windows systems), so if you need immediate and constant feedback, you can try 100-500.
NOTE: The file extension of .swm on the second argument is REQUIRED, the Windows installer only looks for install.wim or install.swm, if you typo it as .wsm as I did the first time, it will boot into the installation only to complain that it can't find the files it needs to proceed.
wimsplit $TEMP/sources/install.wim $USB_DRIVE/sources/install.swm 3000
or for Windows
Dism /Split-Image /ImageFile:"$env:TEMP/sources/install.wim" /SWMFile:"$env:USB_DRIVE\sources\install.swm" /FileSize:3000
Once the split is complete, you can eject your USB drive and WAIT patiently for it to safely eject, since I'm guessing you don't want to corrupt your installer and have to start over from the formatting step again. You can also delete the $TEMP/sources/install.wim if you want, but since we used the system's temporary directory, the file should get deleted the next time you restart.
Now you should be able to put the USB into the computer you want to install Windows on and turn it on and once the logo shows press F12 or F9 or F10 or whatever key your BIOS directs you to press to get into the boot selection menu. Pick the Legacy or UEFI option corresponding to your USB drive (it may just show the manufacturer and not the Win10-1903 name we gave it).
@BigTEvans that likely means your system didn't set the $TEMP variable. Try
export TEMP=/tmp/and then run it again and see if you still get errors.