Okay, so, you are trying to install Windows on a laptop or something for your friend. But you use Linux, and don’t have a Windows box easily accessible. You’re armed with a USB drive, a Linux system, a Windows ISO, and little more. Now what?

I’ll keep this one brief, because if you’re trying to do this, you probably just want to get the damn thing working. And, like me, you are quite possibly already annoyed at why the simple solutions don’t work.

If you are not using Linux and just came here because you got the error:

Windows could not prepare the computer to boot into the next phase of installation

then just skip straight to step 6.

Depending on what you’ve tried so far, you may be in any one of these stages:

All right, so here’s what you have to do:

  1. Format your USB drive as a GPT disk with two partitions, one FAT32 with GPT code ef00, the other NTFS with GPT code 0700. Roughly: 
    $ gdisk /dev/myusbdrive
o
n
<enter>
<enter>
+1G
ef00
n
<enter>
<enter>
<enter>
0700
w
$ sudo mkfs.fat -F32 /dev/myusbdrive1
$ sudo mkfs.ntfs -f /dev/myusbdrive2
  2. Mount the Windows ISO and the partitions: 
    $ sudo mkdir /tmp/iso
$ sudo mount -o loop windows10-enterprise-1909-x64.iso /tmp/iso
$ sudo mkdir /mnt/{win,boot}
$ sudo mount /dev/myusbdrive1 /mnt/boot
$ sudo mount /dev/myusbdrive2 /mnt/win
  3. Copy over all the files to the NTFS partition. Copy over everything except sources to the FAT32 partition. Finally, copy over sources/boot.wim to the FAT32 partition. You’ll get some warnings about copying permissions, but you can ignore those. 
    $ sudo rsync -av /tmp/iso/ /mnt/win/
$ sudo rsync -av --exclude sources /tmp/iso/ /mnt/boot/
$ sudo mkdir /mnt/boot/sources
$ sudo cp -a /tmp/iso/sources/boot.wim /mnt/boot/sources/

    This is this trick, where we work around the FAT32 size limitations by using the FAT32 partition as a “launch pad” into the NTFS partition (which can have large files).

  4. Unmount the USB drive 
    $ sudo sync
$ sudo umount /mnt/boot
$ sudo umount /mnt/win
  5. Plug it into the machine you want to install Windows on. You probably want to reset the BIOS while you’re at it and enable secure boot (in “setup” mode). At the very least, make sure you boot in UEFI mode (not “legacy”). Boot from the USB drive — it should start the installer.

  6. Now we’re going to move the installer onto the drive we’re installing onto! This is a sneaky trick to work around the Windows installer seemingly getting very confused on UEFI systems when there is more than one drive (like here, where our USB drive is inserted). We’re basically going to replicate the install disk onto the internal disk, and then run the installer from the internal disk without the USB drive inserted. We want to do so in such a way that the final install isn’t left with a bunch of empty space at the beginning of the disk (which is hard to reclaim), so we’re going to place the NTFS partition at the end. We also want to combine it with the split partition trick in case the machine cannot boot NTFS partitions.

    Here we go: press Shift + F10 to open a command prompt, then:

    > diskpart
list disk
select disk 0 REM this should be your internal disk
list part
REM this _clears_ the install disk, make sure you want to do this!
clean
REM now, we re-create the two-partition trick on the internal drive:
REM X below is in MB; use the size of the disk - 10GB
create part primary size=X
create part primary
select part 2
format fs=ntfs quick
# in case your disk is large, FAT32 partitions have a size limit:
select part 1 REM this should be the big partition
delete part
create part primary size=10000
select part 1 REM check that this is still the big partition
format fs=fat32
REM now, we need to format them
list volume
REM note down the drive letter for the USB drive.
REM we'll assume it's C: below
select volume 1 REM the volume for the FAT32 partition
assign letter=f
select volume 2 REM the volume for the NTFS partition
assign letter=n
REM all done
exit

    Now we need to set up the files like before

    > xcopy /H /E C:\ N:\
> copy C:\autorun.inf F:\
> copy C:\bootmgr F:\
> copy C:\bootmgr.efi F:\
> copy C:\setup.exe F:\
> xcopy /H /E C:\boot F:\boot REM answer D
> xcopy /H /E C:\efi F:\efi REM answer D
> xcopy /H /E C:\support F:\support REM answer D
> F:
> mkdir sources
> copy C:\sources\boot.wim F:\sources\
> exit

    Phew, okay, now, click the little X in the installer to exit. When the computer reboots, unplug the USB drive.

  7. We’re almost done. When the installer starts, proceed normally until you get to the place where you’re given the option of a “Custom” install. Choose it. When you’re given the partition manager, select the partition at the start of the disk, and delete it. Then, select the “unallocated space” as the installation target.
  8. Finish installation (it should hopefully succeed!).
  9. To reclaim the final bit of space, open up an Administrator command prompt (Windows key → cmd → Ctrl + Shift + Enter), and run: 
    > diskpart
select disk 0 REM make sure this is the disk with the NTFS partition
list part
select part N REM N should be the 10G NTFS partition
delete part
list volume
select volume M REM M should be the volume immediately before N
extend
exit
  10. Enjoy?

If this worked for you, or if you had to do something differently, please let me know on Twitter or by e-mail so I can update this accordingly :)