[syslinux] Err: "Missing OS" Using altMBR.bin

[groups, freeman]

Hi, I'm trying to use the altmbr.bin that came with 3.80 on an ATA IDE 
drive.

When I boot I get this error msg:
> Missing operating system.
>
> No bootable devices--strike F1 to retry boot, F2 for setup utility.
The first line is coming from syslinux, the final line is my BIOS 
offering for me to retry.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

         PC: Dell D500 notebook, P4 1.4GHz
    PC BIOS: Custom by Dell, (c) Dell & Phoenix Technologies 1998, ver=A10
      Boots: USB Flash (tested much), USB IDE, CD, ATA, etc - Nice 
boot-device handling overall
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

My working & booting syslinux 3.80 setup, pre altmbr:

        MBR: from syslinux 3.80
ldlinux.sys: from syslinux 3.80

hda1 -  24G - [0c] -        FAT32 - WinXP (Encrypted via TrueCrypt)
hda2 -  53G - [0f] -        Extended
hda5 - 0.5G - [06] - [boot] FAT16, boots syslinux menu
hda6+ --- Various_Other_Partitions, none with [boot] active ---

AltMBR Purpose: I want to employ the altmbr to have hda5 always be the 
boot partition that syslinux loads from, in a setup where hda5 will 
*not* have the boot flag set, but hda1 *will*.

So to test altmbr I:
- boot into linux and do: dd if=altmbr.bin of=/dev/hda bs=1 (440 records 
are output)
- via fdisk: affirm the boot flag for hda1
- via fdisk: remove the boot flag for hda5
- via DOS (ye olde Norton Disk Editor no less): I edit the 440th byte of 
the MBR (that is, the first byte of the 4-bye disk signature field) to 
contain 0x05

and then I reboot, but get error msg noted above.

FWIW in my testing I tried putting 0x00 thru 0x08 into not only that 
440th byte (offset 439) but concurrently into bytes 438, 439 and 441 for 
each of my test boots, just to moot any offset confusion on my part. 
Always the same error msg tho.

Does anyone have any suggestions of further testing or techniques I 
should do here?

Cheers & thanks for any info!