WinUSB is old, obsolete, and outdated. It can cause problems on newer systems. You should be using WineUSB or other software in place of WinUSB.

This answer is, however, left here as-is for historical purposes.

Create a bootable Windows USB (Vista and above) from Ubuntu through WinUSB software.

Ubuntu 12.04 through 15.04

Run the below commands on terminal to install WinUSB from a PPA,

sudo add-apt-repository ppa:colingille/freshlight
sudo apt-get update
sudo apt-get install winusb

Warning for Ubuntu EFI:

installing WinUSB on EFI loaded Ubuntu will uninstall the grub-efi packages in order to install the grub-pc packages. It will make your system unbootable if you don't manually reinstall grub-efi package before rebooting.

To do the manual re-install do:

sudo update-grub
sudo grub-install /dev/sda
sudo update-grub
sudo reboot

Any Ubuntu version

even other Linux distros as long as GParted and GRUB are installed.

Install GParted, GRUB, 7z, and NTFS on Ubuntu with:

sudo apt-get install gparted grub-pc-bin p7zip-full ntfs-3g

For BIOS: MBR partition scheme

1. Using GParted, rewrite the USB drive's partition table as msdos, format it as NTFS, and then "Manage flags" and add the boot flag.

2. In GParted, right click the USB partition and select Information. Copy the UUID somewhere as you will need it.

3. Mount your Windows ISO or DVD and copy all its files to the USB drive.

4. Go to the USB drive, and if the folder named boot has uppercase characters, make them all lowercase by renaming it.

5. Install GRUB on the USB drive.

   In the below command, replace /dev/sdX with the device (e.g. /dev/sdb, not /dev/sdb1) and replace <USB_mount_folder> with the folder where you mounted the USB drive (which could be like /media/<username>/<UUID>).

   sudo grub-install --target=i386-pc --boot-directory="/<USB_mount_folder>/boot" /dev/sdX
   

6. Create a GRUB config file in the USB drive folder boot/grub/ with the name grub.cfg.

   Write this into the file, replacing <UUID_from_step_2> with the UUID you copied down in step 2.

   echo "If you see this, you have successfully booted from USB :)"
   insmod ntfs
   insmod search_fs_uuid
   search --no-floppy --fs-uuid <UUID_from_step_2> --set root
   ntldr /bootmgr
   boot
   

7. Unmount the USB drive.

8. Now to use it, restart your PC, and boot from the USB drive.

For UEFI: GPT partition scheme *

* Older Windows versions / editions may not be properly supported or not supported at all. I suggest reading the Microsoft UEFI Firmware page.

1. Using GParted, rewrite the partition table of the USB drive as GPT.
2. Create a new primary partition and format it as FAT32.
3. Mount your Windows ISO or DVD and copy all its files to the USB drive.
4. Look on USB in the efi/boot/ folder. If there's a file bootx64.efi (bootia32.efi) then you're done. The USB is bootable. Skip to step 7.
5. Otherwise, open sources/install.wim with the Archive Manager (you must have 7z installed) and browse to ./1/Windows/Boot/EFI/. From here extract bootmgfw.efi somewhere, rename it to bootx64.efi (or bootia32.efi for supported 32 bits OS [?]) and put it on USB in efi/boot/ folder.
6. If you're making a Windows 7 USB, copy the boot folder from efi/microsoft/ to efi folder.
7. Don't forget to unmount (safely remove) the USB drive.
8. Select the proper EFI loader from your BIOS.

Source: My blog post about this can be found at Make a bootable Windows USB from Linux.

Note

When properly used with a compatible target operating system, both of these methods should get you a bootable USB drive. However this does not guarantee successful installation of Windows.

Writing ISOs with WoeUSB (WinUSB fork)

Some answers are outdated, since WinUSB is not working anymore. But there is a working fork called WoeUSB.

Github: https://github.com/WoeUSB/WoeUSB

TLDR:

sudo woeusb --target-filesystem NTFS --device /path/to/your.iso /dev/sdX

Installation

It does not uninstall grub-efi anymore!

☞ Ubuntu / Debian

sudo add-apt-repository universe # contains the p7zip-full dependency
sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt update
sudo apt install woeusb

☞ Arch  

pacaur -S woeusb

☞ Fedora

dnf install -y WoeUSB

☞ OpenSUSE

zypper install WoeUSB

Identifying the USB stick (the /dev/sdX path)

GUI approach

Search for a programm called disks, or if you use gnome you can launch it by executing gnome-disks.

Command line approach

There are several commands available to list storage devices. You might try one of these:

sudo lsblk --scsi --paths 
sudo lshw -class disk -short

Identify your usb device and see which path it has (like /dev/sdX).

Writing the ISO

After installation, write the windows ISO to your storage device with the following command. In the command below replace the X in /dev/sdX with your usb device path (see above how to find it).

sudo woeusb --target-filesystem NTFS --device /path/to/your.iso /dev/sdX

The --target-filesystem NTFS flag is required if the installation image is greater than 4GB, which is the case for the current Windows 10 official ISO file.

WoeUSB is a tool for creating a bootable USB flash drive used for installing Windows. Native UEFI booting is supported for Windows 7 and later images. WoeUSB is an updated fork of the WinUSB project.

Some third-party installers feature Windows installation images (/sources/install.wim) greater than 4GB making FAT32 as target filesystem impossible. NTFS filesystem support has been added to WoeUSB 3.0.0 and later.

WoeUSB (GUI+CLI) installation

Ubuntu 18.04 and later

1. Visit the WoeUSB GitHub page and look for a file with the form woeusb-x.x.x.bash (example: woeusb-5.2.2.bash).

2. Download woeusb-x.x.x.bash (example: wget https://github.com/WoeUSB/WoeUSB/releases/download/v5.2.2/woeusb-5.2.2.bash but the latest version may be different from 5.2.2).

3. Right-click the woeusb-x.x.x.bash file that you downloaded, select Properties, and under the Permissions tab put a check in the checkbox before where it says Allow executing file as program.

4. Open the terminal and change locations with cd to the directory that contains woeusb-x.x.x.bash.

5. Install wimtools.

   sudo apt install wimtools
   

6. Insert a USB flash drive. If there are any files on the USB flash drive back them up to a different device. Open the disks application and make a note of the Device to be used later in step 7. The device should have the form /dev/sdX where X is a letter of the alphabet, and in step 7 you must replace the X with the correct letter of the alphabet that you found in Disks.

7. Run the woeusb-x.x.x.bash script.

   sudo ./woeusb-x.x.x.bash --target-filesystem NTFS --device Windows10.iso /dev/sdX # for Windows 10

   or

   sudo ./woeusb-x.x.x.bash --target-filesystem NTFS --device Windows11.iso /dev/sdX # for Windows 11

   The command for other versions of Windows has a similar form to the above two commands.

The USB drive with Windows installer boots in either UEFI mode or BIOS mode.

• UEFI mode (also boots with Secure Boot enabled)

• BIOS mode (may be called CSM or Legacy mode)

Ubuntu 14.04-18.04

sudo add-apt-repository ppa:nilarimogard/webupd8 
sudo apt update  
sudo apt install woeusb

This will install the WoeUSB graphical interface and the WoeUSB command line tool. WoeUSB supports both UEFI and BIOS for FAT32/NTFS/ExFAT USB flash drives.

To install the WoeUSB command line tool snap package in all currently supported versions of Ubuntu open the terminal and type:

sudo snap install --edge woe-usb  
sudo snap connect woe-usb:removable-media

To launch the woe-usb snap package command line tool run the following command:

/snap/bin/woe-usb.woeusb

If you get a permission denied error click the Permissions button on the woe-usb screen in Ubuntu Software and toggle the permissions options from OFF to ON as shown in the below screenshot.

The WoeUSB GUI is easier to use than the WoeUSB command line tool. Click the radio button to the left of where it says From a disk image (iso), browse to the location of the Windows .iso file, under Target device select a USB flash drive, open Disks application and check that the Device name in Disks matches the Target device in WoeUSB (it should be something like /dev/sdX where X is a letter of the alphabet), and click the Install button to install to create a bootable Windows installation media on the USB flash drive.

Windows USB drive from Ubuntu failing repeatedly
WoeUSB Issues

The current UNetbootin boot chain is not compatible with UEFI and computers that come with a pre-installed copy Windows 8

You can use dd instead, while being careful in what you are doing:

sudo dd if=/path/to/iso/windows.iso of=/dev/sdX bs=4M; sync

• Replace sdX with the drive you want to use (in my case, sdg):
• This requires that your motherboard is able to boot from CDROM-USB.

If you want still to use UNetbootin, there are 2 (3) things that you will need:

1. Unetbootin
2. Gparted
3. Internet access to install all the above, the Windows ISO image and a USB stick with more than 4GB.

So, first, backup all the contents of your usb stick. Once that is done install gparted and unetbootin:

sudo apt-get install gparted unetbootin

Now look for gparted in the Dash or type gparted in the terminal. Select your USB stick from the right dropdown list. In my case it's /dev/sdg, yours may be different. Remove all partitions and create a single big FAT32 partition with Gparted.

Once that is done, unplug and plug your USB stick so it gets mounted (you can also mount it from the same GParted), now execute Unetbootin, again, you can look in the dash or typing in the terminal. Select that you want to use an iso, look for the path your ISO is.

Mark the checkbox to see all devices, here you have to select the very same device you selected in Gparted, otherwise your data can be lost. Select continue. Wait for a moment and done. Restart your pc and select to boot from the USB.

In Non-UEFI machines, we can use GRUB2 to make USB stick bootable. Then, we can use 'ntldr' command in the GRUB2 to boot Windows from USB.

• Enable the boot flag on the target partition of the USB drive. It can be easily done with the use of the tool called "GParted". It is a GUI tool for drive partitioning.
• If the installation image is an ISO file, mount it and access the files.
• Copy all the files to root of USB drive.

• Install GRUB to USB drive:

  sudo grub-install --boot-directory="/media/user/MyUSBDrive/boot" /dev/sdX
  

• Configure GRUB to boot Windows by placing the following file as "/boot/grub/grub.cfg" in the USB drive:

  set menu_color_normal=white/black
  set menu_color_highlight=black/light-gray
  menuentry 'Install Windows 8' {
   ntldr /bootmgr
  }
  

See complete answer at my blog Creating a bootable windows USB from Linux

A simple 'Do it yourself' method

A rather simple 'Do it yourself' method is described at the following links. You can create Windows install drives that work in UEFI mode as well as in BIOS mode,

• help.ubuntu.com/community/Installation/iso2usb

• help.ubuntu.com/community/Installation/iso2usb/diy

• help.ubuntu.com/community/Installation/iso2usb/diy/windows-installer-for-big-files

  This method will work also with [new] versions of Windows 10, where there is a file, install.wim, with a size > 4 GiB, so that the FAT32 file system cannot manage it, when extracted from the iso file. In this case there will be a small FAT32 partition and a bigger NTFS partition. It is tested with an early version of Windows 11, and works there too.

This 'Do it yourself' method is for you

• if you have a Windows iso file that contains a file, install.wim, with a size > 4 GiB, or
• if you don't like PPAs, or
• if you want to 'Do it yourself' and understand the details

mkusb version 12.5.6 and newer versions

This 'Do it yourself' method is implemented in mkusb-tow and available via mkusb version 12.5.6 (mkusb-dus) and mkusb-plug.

You get/update this new version of mkusb from the mkusb PPA via the following commands

sudo add-apt-repository universe  # this line only for standard Ubuntu

sudo add-apt-repository ppa:mkusb/ppa
sudo apt-get update
sudo apt-get install mkusb mkusb-plug

sudo apt-get install usb-pack-efi  # only for persistent live drives

• mkusb 12.5.6 - brief description
• mkusb - general description and manual
• mkusb-plug - description and manual

mkusb-nox and mkusb version 12 (old method for 32-bit systems)

This 'mkusb' method is for you

• if you run a 32-bit operating system, and other methods have problems that extracted files are truncated
• if you want to create a boot drive for Windows 7 or 8
• if you want to create a boot drive for [older] Windows 10 versions, where no file in the iso file exceeds 4 GiB.

It was difficult to find a linux tool that can create boot drives (USB sticks, memory cards ...) with Windows, so I added this feature to mkusb-nox and later on created mkusb version 12 with this feature. It works in all current versions of Ubuntu (and Ubuntu flavours: Kubuntu, Lubuntu ... Xubuntu) and with Debian 8-10. The created boot drive can boot 64-bit Windows in both UEFI and BIOS mode.

• help.ubuntu.com/community/mkusb#Windows_USB_install_drive

dus with guidus alias mkusb version 12:

winusb from the accepted answer is the only easy method I found.

However, there is no winusb package for saucy. You can however install the raring package by downloading it here and opening it with the software installer. It works with saucy.

http://ppa.launchpad.net/colingille/freshlight/ubuntu/pool/main/w/winusb/

You can use WinUSB for that to install WinUSB on your Ubuntu follow these instruction.

Okey, if you are from Ubuntu 13.10,13.04,12.10,12.04, then run this in terminal:

sudo add-apt-repository ppa:colingille/freshlight
sudo apt-get update
sudo apt-get install winusb

and if you are from Ubuntu 14.04 then run this in terminal:

sudo add-apt-repository ppa:colingille/freshlight
sudo sh -c "sed -i 's/trusty/saucy/g' /etc/apt/sources.list.d/colingille-freshlight-trusty.list"
sudo apt-get update
sudo apt-get install winusb

WinUSB comes with a simple GUI with minimal options to go with, here is how to use WinUSB to make bootable Windows USB from Ubuntu. You can use any Windows ISO may be for XP, Windows 7, Windows 8, Windows 8.1 or any other.

1. Insert Flash Drive & Get your Windows ISO (I used Windows 10 Technical Preview) or insert the Windows CD/DVD
2. Start WinUSB and, nothing else really needs to be explained.
3. Select your Source, either ISO or CD Drive
4. Pick your Target (USB) Device. If it doesn’t appear, hit refresh and make sure it’s mounted.
5. Click on “Install” and enter your Password (required to mount devices and write directly to drives)

This is all you need to do to create a bootable Windows USB Stick

Source : How to install and use WinUSB in Ubuntu

1. Install VirtualBox and the VirtualBox Extension Pack found there.

2. Download the Windows image you want to install.

3. Create a Windows virtual machine, and set it up using the Windows ISO file you downloaded.

4. Plug in your USB drive, and expose it to the VM using the latter's Devices/USB menu.

5. Run Microsoft's Media Creation Tool in the VM (you will only be directed to the right page if you access it from Windows). Use this tool to create a bootable Windows USB.

The advantage of this method is that it depends only on Microsoft's tool, which does the configuration for you. The latest Windows images contain a file above FAT32's size limit, which makes it a pain to create a bootable USB drive by hand. My WoeUSB-created drive wasn't recognized by my Dell XPS's UEFI.