7:10 a.m.
Credit where credit is due.. This is a great and well written document,
thanks to the writers and the people who are publishing it
The only sections i'd like to see added is "Set your BIOS disk mode to LBA"
to prevention (which works for some ppl, very new bios'es only have auto/chs
though), and i would also love to see a small chapter on "How did this bug
ever happen" (From what i've been able to gather, it is caused by changes in
the 2.6.x kernel where it was decided HD layout translation should be
handled in user space (which obviously we didn't do..) which causes this
problem)
----- Original Message -----
From: "Jack Aboutboul" <jaboutboul(a)speakeasy.net>
To: <fedora-devel-list(a)redhat.com>
Sent: Tuesday, May 25, 2004 23:25
Subject: Prevention and Recovery of XP Dual Boot Problems
This is the whole document for those who didnt get it.
<---BEGIN--->
Dual Booting Issues With Fedora Core 2 and Windows: Prevention & Recovery
NOTICE: Please read this document in its entirety.
This guide was inspired by the solution developed by Radu Cornea and
Alexandre Oliva in this thread:
http://www.redhat.com/archives/fedora-test-list/2004-May/msg02114.html .
This guide aims to integrate the original solution with the refinements
evolved in that thread. This guide offers an explanation of why the
refinements are beneficial and some workarounds to problems that may
prevent
the uninitiated from using the solution. It also provides a means of
preventing the problem entirely.
Primer:
There is a bug in Fedora Core 2 that causes the hard disk geometry
as
reported in the partition table to be altered during installation.
This
change may cause Windows boot failure. Although this bug is severe, it is
recoverable and no data should be lost. It is important not to panic if
and
when this happens so you do not cause further problems or cause
actual
loss
of data in the process of recovering from the error.
Prevention:
This bug can be avoided entirely by using some preventative steps
while
installing Fedora Core 2. Thanks go out to Cero
(cero(a)coolnetworx.net)
for
discovery and testing of this solution.
To avoid the hard disk geometry to be altered you may enter it
manually
during installation by using the hdN=<drive geometry> parameter
(where N
is
the letter representing the drive with the MBR you will use). To
discover
the current geometry before installing Fedora Core 2 you should use a
utility that can read the drive geometry as reported in the partition
table.
It is important to understand that some tools may not be reporting
the
actual data from this location, but, rather, some derived value, so your
surest way is to use the fdisk utility. You can get this information by
following these steps.
Note: This example will assume you are looking at /dev/hda, which is the
master on the primary IDE interface. If your MBR is located on another
device you should use its name (eg: /dev/hde )
Download and burn the Fedora Core 2 Rescue CD.
Boot from the Rescue CD (there is no need to start networking or mount
drives)
Issue the command: fdisk -l /dev/hda to print the current partition
table
to screen in non-interactive mode.
Write down the drive geometry as reported at the beginning of the output
from fdisk. This is reported as number of Cylinders, Heads, and Sectors
(hence the name CHS).
You can now reboot the computer by simultaneously holding down the keys
Ctrl-Alt-Delete.
You can now boot the Fedora Core 2 installation CD. At the first menu
prompt
you should now choose to run the installer with the known geometry.
Example: linux hda=14593,255,63
The installer should now run normally and not alter your partition
table
geometry entry. If, for any reason, this geometry should be changed
regardless of this preventative step, please use the recovery steps to
correct the geometry of the drive as reported by the partition table.
Recovery:
You have installed Fedora Core 2 and find that you cannot boot
Windows.
Typically the boot process will terminate with the words
Rootnoverify(hd0,0)
Chainloader +1
These are the boot parameters from your Grub configuration. The
parameters
are likely to be correct, but Windows fails to boot because Fedora
Core 2
altered the hard disk geometry as reported by the drives partition table.
IMPORTANT: Do not panic and do not begin using multiple tools in an
attempt
to correct this error. Automated tools can be very dangerous. The
actual
changes that need to be made are minor and benign. By running 3rd party
applications to recover a bootable Windows installation may cause you to
lose your data. You have been warned.
For those who are technically inclined I include here a brief
explanation
of what is going on. The drive has not been damaged and your
partition
table is fine. The problem is that Windows demands a "sane" CHS table.
This table has been altered by Fedora Cores installer and Windows hangs.
Luckily, the actual table, in LBA format, is not corrupted. For those
seeing a strange partition table, take note that you are probably looking
at
the table in CHS values and these values are derived from the
geometry.
The
GNU/Linux operating system does not use these values and operates
purely
with LBA values. Windows should not be using CHS either, but for some
reason it at least checks this geometry and can be prevented from booting
by
them being bad. Changing the drive geometry changes the CHS
partition
table
because this is a virtualization of the true state of affairs on the
drive
which are best described as being mystical. Think of CHS geometry as a
compass. If you change the geometry you have recalibrated where the
needles reference point is and you are no longer looking at true north.
The solution to this problem is very simple, but it may confuse
people
because most people will question why they are seeing strange values
reported from their partition table in CHS format. If you do not trust
this
solution or your ability to follow these steps then you should stop
and
seek
hard disk recovery consulting services. The Fedora Project is in no
way
liable for any data loss and this guide is offered without guarantees.
You
are taking responsibility for what happens. Now, let us go through
the
solution.
Because only the drive geometry is altered there is no need for
manual
intervention in the form of discovering and entry of partition
information.
The information in your partition table is correct. However, you
need to
alter the geometry entry and normally this would require you to re-enter
the
partition table by hand using a tool like fdisk. This is where the
application sfdisk comes to the rescue. Sfdisk can be very powerful in
non-interactive mode, it can output information that can be used as input
elsewhere, and it can accept data as input at run-time. This makes sfdisk
ideal for this solution because you can ask it to read the partition table
and deliver the result in a way that itself can write back when you tell
it
to change your drive geometry. This makes the process fast and less
prone
to human error as very few values need to be supplied. The solution can
be
summed up in a single line with two commands:
sfdisk -d /dev/hda | sfdisk --no-reread -H255 /dev/hda
So that the reader may better understand what is going on here,
lets go
through what each section does and what the parameters mean.
sfdisk -d /dev/hda
This part runs sfdisk non-interactively and dumps the partition table in a
format that sfdisk can also use for input (as we are doing). Try this
command by itself to see your partition table as it is very safe. You
will
want to check to check for warnings in the output. Warnings pose a
problem
because they interfere with the use of this data as input. Output
containing
a warning may look like the example below:
$ sfdisk -d /dev/hda
Warning: extended partition does not start at a cylinder boundary.
DOS and Linux will interpret the contents differently.
# partition table of /dev/hda
unit: sectors
/dev/hda1 : start= 63, size= 16771797, Id= 7, bootable
/dev/hda2 : start= 16771860, size=217632555, Id= f
/dev/hda3 : start= 0, size= 0, Id= 0
/dev/hda4 : start= 0, size= 0, Id= 0
/dev/hda5 : start= 16771923, size=104856192, Id= 7
/dev/hda6 : start=121628178, size=112776237, Id= 7
For reasons unknown, using the option -- quiet does not suppress all
warnings so it becomes the task of the user to discover a way to still use
the output as input. The simplest way is to write the output to a plain
text file, editing out the warning in that text file, and using the edited
text file as the input, thus:
sfdisk -d /dev/hda > MyPartitionTable.txt
editing MyPartitionTable.txt to remove the warnings, saving the
edited
text, and
cat MyPartitionTable.txt | sfdisk --no-reread -H255 /dev/hda
The output from "sfdisk -d /dev/hda" should begin like this (this is the
edited version of the example given before):
# partition table of /dev/hda
unit: sectors
/dev/hda1 : start= 63, size= 16771797, Id= 7, bootable
/dev/hda2 : start= 16771860, size=217632555, Id= f
/dev/hda3 : start= 0, size= 0, Id= 0
/dev/hda4 : start= 0, size= 0, Id= 0
/dev/hda5 : start= 16771923, size=104856192, Id= 7
/dev/hda6 : start=121628178, size=112776237, Id= 7
Note that "cat MyPartitionTable.txt" takes the place of "sfdisk
-d
/dev/hda" as these are now equivalent. In this case the warning portion
has
been stripped, preserving the needed data used by sfdisk in step two
of
the
command.
sfdisk --no-reread -H255 /dev/hda
This portion of the two-part command performs the actual change to
your
hard disk. This main operation is in -H255. This tells sfdisk to
write a
head count of 255 into the drive geometry. This command executed by
itself
would ask for user input of the partition table (just like fdisk).
However,
by piping the table we just read in the first command, this is
avoided and
work is saved and we know the data is correct (or, at least, unchanged).
This is why sfdisk is used.
The --no-reread option allows the command to run even when the
disk has a
mounted partition. Some users may find they need to further force
the
operation to complete. This is done by using --force (sfdisk --no-reread
--force -H255 /dev/hda).
In this example we are only changing the number of heads in the
geometry.
If you know the correct number of cylinders before the Fedora Core
installation changed these values you may also write back this number.
An
example with 14,593 cylinders is provided below.
sfdisk -d /dev/hda | sfdisk --no-reread -H255 -C14593 /dev/hda
The number of reported sectors (S) should not have changed and
remained as
63.
This is the part most likely to be met with the question "if I
change
the
number of heads, must I not also change the the number of
cylinders?" The
answer to this question is "no." When the geometry was changed the number
of heads changed from 255 to 16 and the number of cylinders was increased
to
compensate. As long as the values are large everything should be ok.
Only
the pedantic need worry about changing the number of cylinders
manually.
If
you do not know the value from before you are best off not supplying
this
number.
By using this method there is no need, and indeed you should not,
run a
program that wipes the MBR (like fdisk /mbr). Doing so will cause
you to
lose the Grub pointer installed in the MBR and you will have to use the
Recovery CD to regain access to your Fedora Core installation.
Updating Grub after installation seems to have no effect on the
drive
geometry as the problem seems strictly limited to the Fedora Core
installer.
Good luck and join us on the IRC at #fedora on irc.freenode.net for any
questions you have or contributions to the community you wish to make.
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