This is how I managed to install Windows 10 on my 250gb expansion card whilst having Manjaro installed on my SSD. I can boot into Windows by inserting the expansion card into my laptop and booting it.
Create a windows 10 USB bootable disk and boot from it. (You can use WoeUSB on Linux or Rufus on Windows)
Once you’re in the Setup program, select your language, time and currency format and input method, and click Next. Click the Install Now button. Enter/Skip your Windows key if prompted, and read and accept the software licence. In the next screen, press “SHIFT+F10” to open command prompt.
Partitioning
diskpart
list disk
select disk x (where x is your disk number 0,1,2,3,… and so on)
clean
(This will format the whole disk)
convert gpt
create partition primary size=500
format quick fs=ntfs label=“Windows RE Tools”
assign letter=“T”
set id=“de94bba4-06d1-4d40-a16a-bfd50179d6ac”
gpt attributes=0x8000000000000001
create partition efi size=100
format quick fs=fat32 label=“System”
assign letter=“S”
create partition msr size=128
create partition primary size=230000 (this is the size of your main Windows partition"
format quick fs=ntfs label=“Windows”
assign letter=“W”
create partition primary size=4096
format quick fs=ntfs label=“Recovery Image”
assign letter=“R”
set id=“de94bba4-06d1-4d40-a16a-bfd50179d6ac”
gpt attributes=0x8000000000000001
list volume
exit
Installing
Type C: and press enter.
Type Dir to check if it’s your Windows 10 bootable pendrive. If not, use diskpart to verify your USB drive volume letter. list volume command in the above script shows the drive letter of your USB drive. (In my case I did not use C: but instead either X: or H: I think.) This should be whatever USB drive has the Windows installer on it)
Now, type the following command in sequence:
md R:\RecoveryImage
copy C:\sources\install.wim R:\RecoveryImage\install.wim
cd X:\Windows\System32
dism /Apply-Image /ImageFile:R:\RecoveryImage\install.wim /Index:1 /ApplyDir:W:\
md T:\Recovery\WindowsRE
copy W:\Windows\System32\Recovery\winre.wim T:\Recovery\WindowsRE\winre.wim
bcdboot W:\Windows /s S: /f UEFI
W:\Windows\System32\reagentc /setosimage /path R:\RecoveryImage /target W:\Windows /index 1
W:\Windows\System32\reagentc /setreimage /path T:\Recovery\WindowsRE /target W:\Windows
Updating to Windows 11
Once you have Windows 10 working, you can upgrade to Windows 11 by downloading the Installation Assistant from Download Windows 11
Source:
Some changes had to be made as noted in the comments