Create bootable usb using DD

This method depends on the iso file being designed as bootable, e.g Kaspersky Rescue Disk or Bitdefender Rescue CD, etc.

Plug in the USB flash drive and list its device name using fdisk

sudo fdisk -l
.
.
.
Disk /dev/sdc: 3.8 GiB, 4009754624 bytes, 7831552 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x052e500d

Device     Boot Start     End Sectors  Size Id Type
/dev/sdc1  *     2048 7831551 7829504  3.8G  c W95 FAT32 (LBA)

Unmount the device partition, e.g. sdc1.

umount /dev/sdc1

Copy the iso. file to the device (sdc) not the partition (sdc1). Note the larger block size (4M, default 512bytes) to speed up the transfer. The usb light will flash during copying. Note this can take a long time depending on iso file size. Be patient. When finished dd displays a summary report of bytes copied.

sudo dd if=filename.iso of=/dev/sdc bs=4M

Finally flush the io buffers and eject

sync

sudo eject /dev/sdc