Installed 5.2RC from CDROM, fails to boot in manager

Kevin Oberman oberman at
Fri Jan 2 08:10:04 PST 2004

> Date: Thu, 1 Jan 2004 20:58:48 -0800
> From: Clint Olsen <clint at>
> Sender: owner-freebsd-current at
> On Jan 01, Kevin Oberman wrote:
> > 
> > Try re-writing the MBR. Use the fixit disk (ISO-2) to bring up th live
> > system. Then enter the command: boot0cfg -o packet -B ad0 
> > 
> > There are other options you might want to use, but this should work.
> > (Read the man page.)
> Thanks for the suggestion.  Incidentally, I did finally get the darn system
> to boot, but I had to allow fdisk to determine the slices itself.  I
> checked my old 4.7 machine, and apparently I didn't pay much attention, but
> it also made 2 unused slices at each ends of the disk with the FreeBSD
> slice in the middle.  I'd sure like to know why the blank slice is required
> at the end of the disk...

Hmm. I don't know that I entirely understand. I can create a single slice
on a disk, but I will probably have a few blocks left at the end due to
fdisk limitations on geometry. This is not actually a slice and can't
really be used. I will admit that I don't completely understand why
this happens, but it has always been the case.

I suspect the initial "slice" you refer to is the 64 blocks reserved by
BIOS. This is a restriction in th BIOS specification and the initial 64
blocks may only be used at great peril. Most systems will not object to
the creation of a "dangerously dedicated" disk which is FreeBSD only and
ignores the specified requirements, but some BIOSes will refuse to boot
such a beast. This is why it is called "dangerously dedicated". fdisk
will support this and there is a hidden option in sysinstall, as well,
that can do this (see the man page).but it is not advised. With disk as
cheap as it is today, the risks of dangerously dedicated disks seem to
outweigh (by a large margin) the value of 32 KB of disk space.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at			Phone: +1 510 486-8634

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