These commands are for making any external drives bootable. I am using my pendrive here and these command are tested on Ubuntu 16.04 and its working well.
Plug in the USB Flash Drive and determine the device it's mounted on with the command:
sudo fdisk -l
This time around it was /dev/sdb1 for me, so I'll use that as my example,
The next step is to copy the 'ISO'. Copy to the device not partition (i.e. copy to 'sdb' not 'sdb1') as shown below:
sudo dd if=filename.iso of=/dev/sdb bs=4M
Perform final steps:
sync sudo eject /dev/sdb