Hey guys,
I had exactly the same problem. This is what I did. I rarely shut down my computer, this time I thought I'll give it a rest. I had Samsung SSD and a WD MyBook Duo both plugged into the USB C ports. Although I have booted with the SSD before no worries, this was the first time with the MyBook Duo. I had the message come up on the boot up screen, I pressed shut down. Booted up again in Safe Boot (hold left shift until Apple logo had loaded half way, release) Got an access to the homescreen. Disabled Wifi and unplugged the MyBook Duo from USB C. Pressed restart and it booted up no worries. I think the culprit here might have been the external hdd. I'm not saying that this is the fix for everyone but if you're having a similar issue and have something plugged into USB C that you've never have before on a boot up, might be helpful to go through the process I did.
Hope somebody finds this useful