ISO images are distributed in ISO 9660 format. The format is for optical discs, which could contain both data and the boot sector, making it bootable. Suppose you burn the ISO images directly into an external drive such as a USB stick or portable hard drive; you can directly boot from it.

ISO images are typically used to distribute operating system installers. The image should be bootable, automatically launching the operating system's installation program upon system boot.

Many Linux tools can be used to create a bootable USB drive from ISO image, and dd is the most universally available. dd is a command-line tool and is installed by default in most Linux distributions, such as Ubuntu, Fedora, CentOS, or SUSE.

Steps to burn ISO to USB drive in Linux:

  1. Launch a terminal application.
  2. Check if your image is in the correct ISO 9660 format and is bootable.
    $ file Downloads/ubuntu-21.04-desktop-amd64.iso 
    Downloads/ubuntu-21.04-desktop-amd64.iso: ISO 9660 CD-ROM filesystem data (DOS/MBR boot sector) 'Ubuntu 21.04 amd64' (bootable)
  3. Check for currently available block devices.
    $ lsblk
    NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
    loop0    7:0    0 55.4M  1 loop /snap/core18/1997
    loop1    7:1    0  219M  1 loop /snap/gnome-3-34-1804/66
    loop2    7:2    0 64.8M  1 loop /snap/gtk-common-themes/1514
    loop3    7:3    0 32.3M  1 loop /snap/snapd/11588
    loop4    7:4    0   51M  1 loop /snap/snap-store/518
    loop5    7:5    0 65.1M  1 loop /snap/gtk-common-themes/1515
    sda      8:0    0   20G  0 disk 
    sdb      8:16   0   20G  0 disk 
    ├─sdb1   8:17   0    1M  0 part 
    ├─sdb2   8:18   0  513M  0 part /boot/efi
    └─sdb3   8:19   0 19.5G  0 part /
    sr0     11:0    1 1024M  0 rom
  4. Insert your USB drive or stick and wait for a few seconds for it to be detected by the system.
  5. Check for the device name of your newly inserted USB drive.
    $ lsblk
    NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
    loop0    7:0    0 55.4M  1 loop /snap/core18/1997
    loop1    7:1    0  219M  1 loop /snap/gnome-3-34-1804/66
    loop2    7:2    0 64.8M  1 loop /snap/gtk-common-themes/1514
    loop3    7:3    0 32.3M  1 loop /snap/snapd/11588
    loop4    7:4    0   51M  1 loop /snap/snap-store/518
    loop5    7:5    0 65.1M  1 loop /snap/gtk-common-themes/1515
    sda      8:0    0   20G  0 disk 
    sdb      8:16   0   20G  0 disk 
    ├─sdb1   8:17   0    1M  0 part 
    ├─sdb2   8:18   0  513M  0 part /boot/efi
    └─sdb3   8:19   0 19.5G  0 part /
    sdc      8:32   1 58.6G  0 disk 
    ├─sdc1   8:33   1  2.5G  0 part /media/user/Ubuntu 20.04 LTS amd64
    └─sdc2   8:34   1  3.9M  0 part /media/user/1079-24A3
    sr0     11:0    1 1024M  0 rom  
  6. Make sure all the partitions of your USB drive is not mounted.
    $ sudo umount /dev/sdc1 /dev/sdc2
    [sudo] password for user:
  7. Copy the ISO image to your USB drive using dd.
    $ sudo dd if=Downloads/ubuntu-21.04-desktop-amd64.iso of=/dev/sdc conv=fdatasync
    5505348+0 records in
    5505348+0 records out
    2818738176 bytes (2.8 GB, 2.6 GiB) copied, 628.961 s, 4.5 MB/s
  8. Check if ISO image successfully copied to your USB drive.
    $ sudo blkid /dev/sdc
    /dev/sdc: BLOCK_SIZE="2048" UUID="2021-04-20-11-16-16-00" LABEL="Ubuntu 21.04 amd64" TYPE="iso9660" PTUUID="af8737a9-1e23-4373-b87a-c8b16199d461" PTTYPE="gpt"
  9. Disconnect your USB drive from your machine.
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