A great little tool to burn ISOs to external USB disks:
Etcher

Use Disk Utility to format the external as Fat 32 (may have to format as Mac OS first).

If Etcher doesn’t see the drive at first, use the gear to turn on “Unsafe” mode which lists all attached disks. Then choose your disk with care. “Etching” the ISO to the disk REPLACES the contents of the disk with the ISO image!!!