"Startup Disk Creator" doesn't see my USB Hard Disk

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

sudo umount /dev/sdb1

then you'll need to use DD to write your ISO to the drive:

sudo dd if=/path/to/file.iso of=/dev/sdb bs=1M

be sure to get the correct path, and change sdb to whatever your external drive is mounted as.