Can my WD Passport become a bootable clone of my MAC HD?

Hi. I’m using CCC to create a bootable clone of my MAC HD using Mojave (OS 10.14.2). My WD device is a My Passport Ultra version via a USB (?2.0) connection.
CCC creates the clone successfully on my WD drive, but it isn’t listed as bootable and won’t become the start-up disc when I try to do that.
Any thoughts? Thanks

WD offer no technical support for it … but there are some articles (can’t see My Passport “Ultra” listed as a compatible boot drive though)