Startup disk

Added an external SSD drive to my iMac. (Thunderbolt 3) using the USB adapter. It has Mojave (OS 10.14.6) installed.

It shows up in the Startup Disk control panel. When I select it, it says my Mac will use it to startup, but it always starts up from the internal drive instead. It actually worked once successfully, so I thought everything was OK.

I've unlocked the panel and tried holding the option key down on restart. No luck.

I've tried two different wired keyboards.

The SSD drive show up on my desktop, but I can't seem to startup to it.

I'm not sure what I've done to create this problem. Any suggestion would be appreciated.

I've attached the specs of my iMac.