Comment 29 for bug 277903

I'm going to repeat here my findings that I originally attached to bug #293083 "USB startup thumb drive fails to boot" since the discovery could point to some users being affected by a BIOS bug.

If I remove the hard drive from the system, rather than just change the BIOS boot order so USB is tried first, The USB device boots successfully.

The "Missing Operating System" message is written by the syslinux MBR code when it can't find the second stage code in the bootable file-system.

In the process of investigating this issue I discovered this boot-device issue which suggests a BIOS bug. The affected Sony Vaio has a Phoenix BIOS.

It suggests that when the syslinux MBR (Master Boot Record) code asks the BIOS to read the first sector from the bootable partition (the file-system's boot-loader code) one of two things might be happening:

1. BIOS passes the wrong drive number to the boot-loader as part of the PnP header (available in DL when execution is passed to the MBR from BIOS along with the PnP header at ES:DI)
2. BIOS ignores the drive number passed (in DL) to int 0x13 (Read sector)