bootable disk: how?

[der Mouse <mouse%Rodents.Montreal.QC.CA@localhost>]

Okay, I give up.  What does it take to make a disk bootable?

I thought I knew.  But either I'm wrong or the tools aren't doing what
I thought they were (and, if I'm reading things right, what they're
documented as).

I've used fdisk -aui and answered y to the "update the bootcode"
prompt.  I've used installboot to update the second-level bootlocks.
I've even tried disklabel -B (this started on a 3.0 system).  Nothing
works.

Most recently, I moved the disk to a 4.0 machine and used fdisk -aui
and installboot there.  It *still* fails to find an OS.  (The exact
message differs from system to system, depending on the BIOS; the 4.0
machine says "DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER",
while the 3.0 machine says "Operating System not found".  Of course, I
have to either unplug other disks or disable booting from them, or the
machine boots normally from wd0 (3.0) or sd0 (4.0).)

fdisk (4.0) shows

Partition table:
0: NetBSD (sysid 169)
    start 63, size 8448237 (4125 MB, Cyls 0-525/224/63), Active
1: <UNUSED>
2: <UNUSED>
3: <UNUSED>
Bootselector disabled.

disklabel (4.0) says

#        size    offset     fstype [fsize bsize cpg/sgs]
 a:   8448237        63     4.2BSD   2048 16384 26296  # (Cyl.      0*-   8939)
 c:   8448237        63     unused      0     0        # (Cyl.      0*-   8939)
 d:   8448300         0     unused      0     0        # (Cyl.      0 -   8939)

The disk is (again, 4.0)

pci0 at mainbus0 bus 0: configuration mode 1
pci0: i/o space, memory space enabled, rd/line, rd/mult, wr/inv ok
piixide0 at pci0 dev 7 function 1
piixide0: Intel 82371AB IDE controller (PIIX4) (rev. 0x01)
piixide0: bus-master DMA support present
piixide0: primary channel wired to compatibility mode
piixide0: primary channel interrupting at irq 14
atabus0 at piixide0 channel 0
wd0 at atabus0 drive 1: <FUJITSU MPD3043AT>
wd0: drive supports 16-sector PIO transfers, LBA addressing
wd0: 4125 MB, 8940 cyl, 15 head, 63 sec, 512 bytes/sect x 8448300 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 4 (Ultra/66)
wd0(piixide0:0:1): using PIO mode 4, Ultra-DMA mode 2 (Ultra/33) (using DMA)

The disk works fine for non-boot purposes; the only symptom I have is
this inability to get the BIOS to boot from it.

I'd even welcome RTFM responses, provided they say which FM to R; I've
read through manpages over and over again, but of course am not immune
to missing things.

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