Making Fedora Core 2 and Windows play well together

This is the whole document for those who didnt get it.
[Original document posted by Jeff Spaleta --LWN ed]

<---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@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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