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Guide to Booting ISO from a USB Stick

Creating a bootable USB drive is a fundamental skill for both IT professionals and tech enthusiasts. In this article, you will learn step-by-step how to create a Windows bootable USB drive on Linux using only terminal commands. Whether you are setting up a new system, repairing an existing one, or just need a reliable portable installer—this guide is for you.

We will cover each step individually: from preparing your USB drive and partitioning it, to writing the Windows ISO file to the USB, all done directly with terminal commands.

The Difference Between Linux ISOHybrid and Windows ISO#

When creating a bootable USB drive, it is crucial to understand the differences between Linux ISOHybrid images and Windows ISO images. Although both serve as installation media, their design, functionality, and use cases differ. These differences affect how the images interact with USB drives and how tools like dd or specialized utilities should be used.

This distinction becomes particularly important when creating a Windows bootable USB in a Linux environment, as this process often requires more steps than working with a Linux ISOHybrid. Let's now delve into the details to better understand these formats and their impact on the process of creating a bootable USB.

This context will help you understand the specific challenges and solutions encountered when working with a Windows ISO on Linux.

What is a Hybrid ISO?#

A Hybrid ISO is a special type of ISO that can function both as a classic optical disc (CD/DVD) image and as a bootable image on devices like USB drives. Traditionally, ISO files are designed to be written to a CD or DVD. Hybrid ISOs extend this functionality; the same file can be written directly to a USB drive without any additional modifications and become bootable.

This dual functionality is possible because it includes both the ISO9660 file system for CD/DVD compatibility and an MBR (Master Boot Record) structure that provides USB boot support.

How to Tell if an ISO is Hybrid#

You can use the following methods to determine if an ISO is hybrid:

  1. Check with the file Command

The file command provides information about the structure of the ISO:

file path/to/image.iso

For a hybrid ISO, the output usually includes the phrase "(DOS/MBR boot sector)".

  1. Examine the MBR with fdisk

Hybrid ISOs contain an MBR to support booting from USB. You can check this with fdisk:

fdisk -l path/to/image.iso

If you see bootable partition information (e.g., a single partition), the ISO is likely hybrid. For a non-hybrid ISO, partition details will not be displayed.

  1. Check the El Torito Boot Record

Hybrid ISOs use the El Torito1 standard. You can verify this with the xorriso tool:

xorriso -indev path/to/image.iso

If the output under the Boot record section contains information about the bootloader and partition table, the ISO is bootable.

Requirements#

Before starting the process, make sure you have the following ready:

  1. Windows ISO file: Download the official ISO file from the Microsoft website.

  2. USB drive: It should have at least 8 GB of free space. Back up any important data as all content will be erased.

  3. Linux terminal access: All commands will be run through the terminal.

Step 1: Identify Your USB Drive#

Plug in your USB drive and open the terminal. Run the following command to determine the device name:

lsblk

In the output, you will typically see a name like /dev/sdX. The letter X represents the drive letter—carefully note the correct drive.

Step 2: Partition the USB Drive#

Before formatting, you need to create the necessary partitions on the USB drive. Use the fdisk tool for this.

To avoid errors during partitioning and formatting, unmount any existing partitions on the USB beforehand:

sudo umount /dev/sdX*

  • Replace /dev/sdX* with the appropriate partitions of your USB drive.

  • Start the fdisk tool:

sudo fdisk /dev/sdX

  1. Follow these steps within fdisk:

  2. Press g to create a new GPT partition table.

  3. Press n to add a new partition.
  4. Press p to create a primary partition.
  5. Create the first partition, leaving a small amount of space for the boot partition (about 1 MB is sufficient).
  6. Allocate the remaining space for the second partition.

In a Windows environment, the operating system on removable drives typically recognizes only the first primary partition. Therefore, the data partition that will be accessible by Windows should be the first, and the boot partition containing system files should be the second.

  1. Set the file system type to Microsoft basic data:

  2. Press t and select type number 11 for both partitions.

  3. Set the bootable flag:

  4. Press x.

  5. Press A to select the partition to be made bootable.

  6. Save changes and exit:

  7. Press r to exit expert mode.

  8. Press w.

Step 3: Format the USB Drive#

Use NTFS for the data partition and FAT32 for the boot partition.

For NTFS:

sudo mkfs.ntfs -f /dev/sdX1

For FAT32:

sudo mkfs.vfat -F 32 /dev/sdX2

Replace /dev/sdX1 and /dev/sdX2 with the correct partitions.

Step 4: Copy Windows ISO Files to the USB#

Mount the ISO file and the USB drive, then copy the contents:

  1. Create mount points:
sudo mkdir /mnt/iso
sudo mkdir /mnt/drive
  1. Mount the ISO:
sudo mount -o loop /path/to/your.iso /mnt/iso
sudo mount /dev/sdX1 /mnt/drive
  1. Copy the files:
sudo cp -r /mnt/iso/* /mnt/drive

This process can take up to 10 minutes depending on your USB speed.

  1. Unmount the ISO:
sudo umount /mnt/iso
sudo rmdir /mnt/iso

Step 5: Write the Rufus Boot Image#

First, download the boot image from the Rufus2 repository:

wget https://github.com/pbatard/rufus/raw/master/res/uefi/uefi-ntfs.img

Use the dd command to write the downloaded uefi-ntfs.img file to the USB:

sudo dd if=/path/to/uefi-ntfs.img of=/dev/sdX2 bs=1M status=progress

Critical Warning; Replace /dev/sdX2 with the correct drive name. If you enter the wrong device name, important data, including your system disk, could be erased!

Step 6: Install the Bootloader#

We will use grub to install the bootloader. The command may vary depending on your distribution.

Note for Debian/Ubuntu Users

If the grub2-install command is not found, your system might be using grub-install. Also, the grub-pc package may need to be installed for the i386-pc target. To install: sudo apt-get install grub-pc

Install the bootloader using the grub2-install (or grub-install) command:

sudo grub2-install --target=i386-pc --boot-directory=/mnt/drive --force /dev/sdX
  • --boot-directory=/mnt/drive: The mount point of the USB's Windows data partition (see Step 4).
  • /dev/sdX: The USB device itself.

Attention! Double-check the drive name before running the command.

Step 7: Test the Bootable USB#

Plug the USB into the target system and set USB as the priority boot device in the BIOS/UEFI settings. If necessary, disable the Secure Boot option.

Common Issues and Solutions#

  • Bootloader Errors: If the USB does not boot, the ISO may require additional configuration.

  • Permission Errors: Always run commands with sudo.

  • Corrupt ISO File: To verify the integrity of the ISO, use this command:

sha256sum /path/to/windows.iso

Compare the output with the official SHA256 value.

Conclusion#

By following these steps, you can easily create a Windows bootable USB on Linux using only terminal commands. This method should always be at your fingertips for new installations, system repairs, or emergencies.

Happy booting! 💻

Resources#

Guide to Booting ISO from a USB Stick

  1. https://en.wikipedia.org/wiki/ISO_9660#El_Torito ↩

  2. https://rufus.ie/en/ ↩

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