Script creates bootable usb from windows iso, tested on Ubuntu 15.04 with Windows 8 and Windows 10 iso files on BIOS and UEFI machines.

winusb.sh

#!/bin/sh

# Requirements: 
# parted - to make gpt and partion
# sfdisk - to make mbr and partition over gpt
# blkid - to get UUID of partition for grub
# grub-pc-bin - files to install grub from UEFI-booted machines to MBR

usb="/dev/sdX" # path to usb device file (without number at end) sudo fdisk -l
iso="/path/to/windows.iso" # path to windows iso
usb_mount="/mnt/usb"
iso_mount="/mnt/iso"
mbr=true # mbr=true creates BIOS bootable, if not, then UEFI bootable


# Find and unmount all partitions of device
UmountUSB () {
    mounts=`mount -l | grep "${usb}" | grep -o "^\S*"`
    for i in ${mounts}
    do
        sudo umount ${i}
    done
}

UmountUSB

if [ "${mbr}" = "true" ]
then
# Create MBR and bootable FAT32 partition starting at 2048 sector.
    sudo parted -s ${usb} mklabel msdos
    sudo parted -s ${usb} mkpart primary fat32 2048s 100%
    sudo sfdisk --activate=1 ${usb}
else
# Create GPT and FAT32 partition starting at 2048 sector.
    sudo parted -s ${usb} mklabel gpt
    sudo parted -s ${usb} mkpart primary fat32 2048s 100%
fi

# Format partition
sudo mkfs.vfat -F 32 ${usb}1

# Mount ISO and USB
sudo mkdir -p ${usb_mount}
sudo mount ${usb}1 ${usb_mount} 
sudo mkdir -p ${iso_mount} 
sudo mount ${iso} ${iso_mount} -o loop

# Copy Windows files
echo "Copying Windows files ..."
sudo cp -r ${iso_mount}/* ${usb_mount} 


if [ "${mbr}" = "true" ]
then
# Install GRUB
    sudo grub-install --target=i386-pc \
    --boot-directory="${usb_mount}/boot" \
    --force \
    ${usb}

# Find partition UUID by blkid
    uuid=`sudo blkid | grep "${usb}1" | sed -n 's/.*\sUUID=\"\([^\"]*\)\".*/\1/p'`

# Create GRUB conf pointing to UUID of partition
    config="${usb_mount}/boot/grub/grub.cfg"
    echo "echo \"Booting from USB\"
    insmod part_msdos
    insmod fat
    insmod ntldr
    search --no-floppy --fs-uuid --set=root ${uuid}
    ntldr (\$root)/bootmgr
    boot
    " | sudo tee ${config} > /dev/null
fi

# Unmount ISO and USB
sudo umount ${usb_mount} ${iso_mount}