MBP retina mid 2012, fresh 10.13 upgrade from 10.11. Didn't try to reboot after installation completed, just shutdown to reset NVRAM – I had issues with connecting to 5GHZ networks for ages and decided to try to fix this.
Now I get a "still waiting for root device" message. And that's on native hardware and internal SSD!
Once booted from external drive I can access internal SSD with no issues.
Any ideas how to debug this?
P.S.: I also can't boot from USB 3 devices so have to use USB 2 ones. Looks like USB 3 support on MBP Retina 2012 laptops is limited.