wufik.sh is Windows USB Flash Installer Kreator.

Usage:
$ sudo bash wufik.sh windows_install.iso /dev/usb_drive

=====

Task that wufik solves is that while in EFI-compatible PCs creating of Windows
installation flash is as simple as `dd win_install.iso /dev/sdb` (EFI finds and
boot NTLDR by self), on legacy BIOS PCs we need to have some external bootloader
to boot installer.

So here are steps to create such flash drive using GRUB2, and wufik automates it.

First of all, part a flash drive (at least 8Gb drive):
- one small bootable partition for GRUB files
- second partition for Windows installer files

sdb is a name of USB device here.

=====

$ sudo cfdisk -z /dev/sdb
                            ┌ Select label type ───┐
                            │ gpt                  │
select dos (MBR) type here->│ dos                  │
                            │ sgi                  │
                            │ sun                  │
                            └──────────────────────┘
=====

Create two primary partitions:
1. 100M is enough for GRUB but I've created 500M there. Type of partition is
Linux (83). Then mark this partition as Bootable.
2. The rest of device. Change type of this partition to HPHS/NTFS/exFAT (7).
Windows 7 installation media doesn't have files bigger then 4Gb so type FAT32(b)
can be used for this partition. But 8, 10, 11, etc Windowses have big files so
NTFS is a relevant choice.


    Device       Boot      Start       End   Sectors   Size  Id Type
>>  /dev/sdb1    *          2048   1026047   1024000   500M  83 Linux           
    /dev/sdb2            1024048  15124991  14100944   6.7G   7 HPFS/NTFS/exFAT


 ┌────────────────────────────────────────────────────────────────────────────┐
 │Partition type: Linux (83)                                                  │
 │   Attributes : 80                                                          │
 └────────────────────────────────────────────────────────────────────────────┘
     [Bootable]  [ Delete ]  [ Resize ]  [  Quit  ]  [  Type  ]  [  Help  ]
     [  Write ]  [  Dump  ]


Write chages to disk and quit.

=====

Next format this partitions:

$ sudo mkfs.ext4 /dev/sdb1
 ....
$ sudo mkfs.ntfs -f /dev/sdb2
 ....
 
=====

Next step is to install GRUB2 to USB flash. We also need to specify a directory
where GRUB2 files and modules will be located.

Let's mount sdb1 to /mnt 
$ sudo mount /dev/sdb1 /mnt

Then install GRUB2. Name of command "grub2-install" can differ in a different
distributions. grub-install is a second candidate to try. Also check /usr/sbin
for variants.

$ sudo grub2-install --boot-directory=/mnt /dev/sdb

=====

Create really small config file /mnt/grub2/grub.cfg

menuentry Windows {
	set root=(hd0,msdos2)
	ntldr /bootmgr
	boot
}

hd0 is a name of boot device here (USB flash).
msdos2 is a name of second partition on this device.

=====

Last step is to copy all files from Windows installation media to prepared
partition.

Unmount sdb1 first
$ sudo umount /dev/sdb1

Mount NTFS partition
$ sudo mount /dev/sdb2 /mnt

Mount Windows .iso image.
$ sudo mkdir /winiso
$ sudo mount ~/Windows-install.iso /winiso

Copy all of this and clean
$ sudo cp -r /winiso/* /mnt
$ sync
$ sudo umount /mnt
$ sudo umount /winiso
$ sudo rmdir /winiso

Note that you can use this flash drive to boot any Windows install. Just remove
installation files from second partition and copy another.

=====

References:
https://willhaley.com/blog/windows-installer-usb-linux
https://askubuntu.com/questions/367011/boot-windows-7-iso-from-grub2