booting development image from USB gets "root mount failed"

[nacho Lariguet]

Mounting root from ufs:part-by-label/DragonFly_v5.9.0.645.gfaeb2.a
no disk named 'part-by-label/DragonFly_v5.9.0.645.gfaeb2.a'
setrootbyname failed
ffs_mountroot: can't find rootvp
Root mount failed: 6

Manual root filesystem specification:
 <fstype>:<device> Specify root (e.g. ufs:da0s1a)
 ?                 List valid disk boot devices
 panic             Just panic
 abort             Abort manual input

mountroot>?
Possibly valid devices for root FS:
 "md0" "md0s0" "vn0" "vn1" "vn2" "vn3"



Attempted to boot latest image (2020-07-28) today
from a Kingston DataTraveller DT-100 G3 16 GB USB key
on a Lenovo ThinkCentre Tiny M715q
- with a Kingston SATA SSD installed
- without the SATA SSD; ie: a diskless-system

The above details were manually typed from the screen
so please spare me any minor typo.