[syslinux] BIOS bug? USB + "Missing operating system"

[TJ]

A follow-up to report my findings using my "mbr-diag" diagnostic MBR
(see Patch "Add Diagnostic MBR for trouble-shooting") to determine why
an external USB flash mass-storage device fails to boot when the PC's
hard disk is attached.

Hard-disk present:

 DIAG L D81h C3FEh HFFh S3Fh P1 O00000020h M0000h E00h

Hard-disk not present:

 DIAG L D80h C3FFh H10h S3Fh P1 O00000020h MAA55h E00h

>From these reports the most obvious observation is that the partition
boot-sector magic-bytes (M0000h) aren't there after reading the boot
sector. That indicates the 'wrong' sector (or device) was used. The BIOS
interrupt didn't report an error (E00h).

The drive number (D81h) indicates the BIOS thinks the USB mass-storage
device is a hard disk.

The other major difference is the number of heads reported. The
successful scenario (hard-disk not present) shows a head count (H10h) of
16 whereas the unsuccessful scenario shows 255 heads.

The cylinder counts are different but only by one: successful is 1023
(C3FFh) versus 1022 (C3FEh).

I'm currently unable to think of a rational explanation for these
differences.