FreeBSD MBRs

[Jerry McAllister]

On Wed, Aug 22, 2007 at 12:27:51PM +0100, Christopher Key wrote:

> Hello,
> 
> I've a machine with 3 SATA drives.  The first (ad8) with a standard
> FreeBSD install in a single slice with /boot/boot0 MBR.  The remaining
> two drives (ad10, ad12) are in a RAID1 mirror with 3 slices, and used
> for storing data.  They have the /boot/mbr MBR.
> 
> After booting off various USB flash drives to try and update the BIOS on
> my machine, it got into a state where during startup, it would display
> 'Missing operating system' and hang.  What seems to have been happening
> is that it was trying to boot from one of my data store drives, despite
> the boot order of the disks set in the BIOS.
> 
> The only solution that I found was to start booting from a USB flash
> drive with a boot0 MBR, and to hit F5 to change to booting from my first
> drive  After this, the machine then reboots quite happily until I hit F5
> again, in which case I get the same 'Missing operating behavior'.  This
> persists even while power cycling the machine.
> 
> I had imagined the boot process to be entirely stateless, certainly
> across power cycles.  The BIOS executes the MBR on the first drive in
> its boot boot.  The boot0 MBR then allowed you to either execute the
> boot sector from any of the slices on the current drive, or to execute
> the MBR from the next drive in the list.
> 
> However, this clearly isn't what's happening.  Is it boot0 remembering
> my F5 key stroke, or is it more likely that the BIOS is remembering
> something?  Does anyone have any recommendations to avoid this in the
> future?  Is putting boot0 on all three drives a good idea perhaps?

I am having a little trouble following the narrative here, but maybe I
can comment on a couple of specific questions.

The system does record your most recent menu choice and the next time
it will be the default if you don't hit another one during the countdown.
This makes it default to trying to boot the way it last booted.

You only need an MBR on disks that will be booted.  I don't know as
it will actually hurt anything to write an MBR on non-boot, data only
disks, but it can garbage up you menu with non-functional choices.

Those other disk with an MBR show up as an F5 and maybe F6, etc (I've
never had more than two at a time) in the menu, but then if you hit
F5, it won't be able to go anywhere, because there is no real boot
stuff on that disk.

////jerry

> 
> Regards,
> 
> Chris
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