Installing 11RC5 on and old AMD64

[Ted Spradley <tsprad%talent-free-studios.com@localhost>]

I have a 2015 vintage HP Pavilion with an early, buggy UEFI. I have
installed NetBSD on this box several times over the years, but now
11RC5 goes through the installation smoothly, all the configuration and
setup, but then won't boot, says there is no operating system. As near
as I can tell, the installation process writes nothing at all to the
EFI partition.

Most recently I had FreeBSD installed on this box. I left the EFI
partition, deleted all the rest (zroot), and created a new swap
partition and a 32GB partition for NetBSD (I want to leave space for
other things). I put the install image on a USB memstick and booted it
as a UEFI device. Should I have deleted the old EFI partition first?
Or should I have booted the memstick installer in legacy BIOS mode? Or
is there something else I'm just not getting?

-- 
Ted Spradley <tsprad%talent-free-studios.com@localhost>