PBR is not bootable

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

AFAICT I have just performed the same operations on 2 identical disks (amd64),
fdisk -i, fdisk -u ..., fdisk -0a, installboot ...

Disk: /dev/rwd0d
...
0: NetBSD (sysid 169)
    start 63, size 976773105 (476940 MB, Cyls 0-60801/80/63), Active
        PBR is not bootable: All bytes are identical (0x00)
1: <UNUSED>

whereas

Disk: /dev/rwd1d
...
0: NetBSD (sysid 169)
    start 63, size 976773105 (476940 MB, Cyls 0-60801/80/63), Active
1: <UNUSED>

and installboot /dev/rwd1d is happy, but

# installboot -v /dev/rwd0d /usr/mdec/bootxx_ffsv1
File system:         /dev/rwd0d
Primary bootstrap:   /usr/mdec/bootxx_ffsv1
installboot: Old BPB too big, use -f (may invalidate filesystem)
installboot: Set bootstrap operation failed

(I tried -f, but nothing changed)

So, how can the difference happen? (Both disks in same computer, so same
BIOS)

Cheers,

Patrick