I have used Startup Disk Creator several times before succesfully creating bootable usb pendrives from iso images.
However, now I'm trying to do the same with a Western Digital external USB portable hard disk (it's a disk proper, one of those that actually spin), and it is not even listed among the available devices in Startup Disk Creator.
What do I have to do in order to make it a bootable drive so that I can boot a live linux distro from it?!?!
You could always do this the command line way.
Unmount the drive first
then you'll need to use DD to write your ISO to the drive:
be sure to get the correct path, and change sdb to whatever your external drive is mounted as.