booting from wrong disk

Polytropon freebsd at
Thu Oct 15 18:03:44 UTC 2009

On Thu, 15 Oct 2009 12:07:17 -0400, PJ <af.gourmet at> wrote:
> I see from the boot process that this should fix it...
> # fdisk -B -b /boot/boot0 ad4
> but...
> how do I get this onto the right disk? If I boot from ad4 or ad12 and
> change the mbr, then it will be the ad12 that will be booting from ad4
> and vice versa... or have I got it wrong? But in the end, I suppose it
> really doesn't matter, or does it?

The "active" flag on the partition tells the MBR loader from
which device to boot. In other words, the FIRST device that
is market as "active" will be booted. If you have, for example,
ad4 ad ad12 in your system, and ad4 is detected first, then
it will be booted, no matter if ad12 is marked "active", too.
Of course, ad4's boot loader can refer to another device as
bootdev (booting device), but that's out of scope for now.

If you have two disks ad4 and ad12, both marked "active", and
you exchange them physically, boot order will change, too.

A similar setting could involve things like a mix of ATA, SCSI
and SATA disks. Again, who comes first will be booted - but
as I said, only if the device is marked "active"; if not, it
will be ignored.

Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...

More information about the freebsd-questions mailing list