This is not the best way to create a bootable drive, especially if the ISO image contains an operating system not recommended for Macs. However, there are ways to do it.
To create a bootable drive from an ISO file with Terminal:
Download the ISO file
Open Terminal and type in hdiutil convert -format UDRW -o /path/to/target.img /path/to/source.iso
Insert your external drive
Type in diskutil list, press Enter and make a note of your drive’s identifier in the list of devices
Type in diskutil unmountDisk /dev/diskN —N should be the number in your disk’s identifier
Run sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m. If you see the error dd: /dev/diskN: Resource busy, go to Disk Utility and unmount the drive to make sure that it’s not being used
Eject your drive and disconnect it from your laptop. You can do it through Terminal by running diskutil eject /dev/diskN