How to create a bootable USB drive from an ISO image on Linux

Creating a bootable USB drive from an ISO image enables operating system installations, live environments, and recovery tools on hardware that no longer includes optical drives. Reliable removable media is essential when deploying new systems, testing distributions, or repairing unbootable machines.

On Linux, an ISO image typically uses the ISO 9660 format and contains both the filesystem content and bootloader data required for booting. The dd utility copies this image sector by sector from a regular file to a block device such as /dev/sdc, preserving the boot sector, partition table, and metadata that make the medium bootable.

Because dd performs raw writes, targeting the wrong device can erase an entire disk with all existing partitions and data. Correctly identifying the USB device, unmounting any of its partitions, and confirming the image type before writing reduces the risk of data loss and produces a USB drive that boots reliably on compatible systems.

1. Open a terminal on the Linux system.
2. Verify the installation ISO image type using the file command.

   $ 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. List the current block devices to understand the existing disk layout.

   $ 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 the USB drive into an available port on the system.
5. Identify the USB drive device name by listing block devices again.

   $ 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

   Match the USB drive by size and removable flag to avoid selecting an internal disk.

6. Ensure that all partitions on the USB drive are unmounted.

   $ sudo umount /dev/sdc1 /dev/sdc2
   [sudo] password for user:

   Unmounting prevents filesystem corruption and guarantees exclusive access for the raw write operation.

7. Write the ISO image to the 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

   Related: How to check the progress of dd command in Linux

   Confirm the target device (for example /dev/sdc) carefully; using the wrong disk erases all existing data irreversibly.

8. Verify that the target device now exposes the expected label and filesystem type.

   $ 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"

   The LABEL and TYPE fields confirm that the image was written correctly as an ISO 9660 filesystem.

9. Disconnect the USB drive safely from the system once writing and verification are complete.

   On desktop environments, use the graphical eject or safely remove option before unplugging the device.