In case anyone else hits this strange and annoying issue…
Scenario: installer written to USB drive (several times, using different OS/tools…); BIOS finds USB drive as bootable, select it, first few icons light up, reaches disk one and drops out to debug with error “Can’t find boot partition”. USB drive with Haiku tested on another computer, works fine.
Switched to different USB socket on (2nd user, Lenovo “edu” laptop) - works fine. Assuming something a bit wrong with the other socket!