NetBSD MBR boot / Error no operating system

[steve%prd.co.uk@localhost (Steve Blinkhorn)]

Machine: Fujitsu Primergy RX1330 M3 server
OS: NetBSD 7.0.1 amd64

I have loaded a full distribution onto the primary disk, having
failed completely to install from installation media on the machine
itself, whether by USB stick, DVD-ROM, or a partable USB drive, by
extracting the disk and using a USB caddy to connect to another
machine (by chance also a Fujitsu), then doing an install using
sysinst.

At the installboot phase, the "Old PBR too big" message appeared, so I
used -f with a manual installboot, after which sysinst proceeded
normally.   I was then able to boot from this disk in its USB caddy
without difficulty on the second machine and run in multi-user mode.

But returning it to the server, it gives the message:

NetBSD MBR boot
Error No operating system

Fujitsu tech support is of the opinion that there is a conflict
between the BIOS and the boot code.   We have enabled CSM (compatibility
support) and set everything to Legacy mode, hardware RAID is disabled,
the disk is first in the boot priority list, and clearly something is
being read from it, else the "NetBSD MBR boot" line would not appear.

Any suggestions?

-- 
Steve Blinkhorn <steve%prd.co.uk@localhost>