mean of fdisk /mbr

NOTE:  fdisk /mbr isn't meant for use with Windows 2000 or XP.


FDISK /MBR Rewrites the Master Boot Record
http://support.microsoft.com/default.aspx?scid=KB;en-us;q69013

The information in this article applies to:

    * Microsoft MS-DOS operating system
    * Microsoft Windows 95
    * Microsoft Windows 98
    * Microsoft Windows Millennium Edition

This article was previously published under Q69013

SUMMARY
The MS-DOS Fdisk utility usually updates the master boot record (MBR) only if no master boot record exists. Repartitioning with Fdisk does not rewrite this information.

Fdisk has an undocumented parameter called /mbr that causes it to write the master boot record to the hard disk without altering the partition table information.

WARNING: Writing the master boot record to the hard disk in this manner can make certain hard disks partitioned with SpeedStor unusable. It can also cause problems for some dual-boot programs and disks with more than four partitions.


MORE INFORMATION

What is the MBR?

At the end of the ROM BIOS bootstrap routine, the BIOS reads and executes the first physical sector of the first floppy or hard disk on the system. This first sector of the hard disk is called the master boot record (or sometimes the partition table or master boot block). There is a small program at the beginning of this sector of the hard disk. The partition information, or partition table, is stored at the end of this sector. This program uses the partition information to determine which partition is bootable (usually the first primary DOS partition) and attempts to boot from it.

This program is written to the disk by the fdisk /mbr command and is usually called the master boot record. During typical operation, Fdisk writes this program to the disk only if there is no master boot record.

NOTE: The fdisk /mbr command only re-writes the MBR on the system drive (DISK-0) using BIOS calls. You cannot specify any other drive for the fdisk /mbr command to operate on other than DISK-0.
Why is the MBR Changed During Setup?
During the installation of Microsoft MS-DOS Upgrade, Setup replaces the master boot record on the hard disk with code to display a message similar to:
The MS-DOS 5.0 Setup was not completed.
Insert the UNINSTALL #1 diskette in drive A.
Press the ENTER key to continue.

This message should be erased and the master boot code rewritten before Setup is completed. If a problem occurs during Setup and you return to the previous version of MS-DOS, Uninstal should also remove this message. However, if Setup or Uninstal does not remove this message, or if the master boot record becomes corrupted, a new master boot record can be written to the disk using the following command:
fdisk /mbr

WARNINGS
Do not use this command if any of the following conditions exists:

    * The disk was partitioned using the Storage Dimensions SpeedStor utility with its /bootall option.
    * More than four partitions exist.
    * Certain dual-boot programs are in use.

The Storage Dimensions SpeedStor utility with the /bootall option redefines the drive's physical parameters (cylinder, head, and sector). The /bootall switch stores information about how the drive has been changed in an area of the master boot record that MS-DOS does not use. The fdisk /mbr command erases that information, making the disk unusable.

Some older original equipment manufacturer (OEM) versions of MS-DOS and some third-party partitioning utilities can create more than four partitions. Additional partition information is commonly stored information on partitions in an area that the fdisk /mbr command overwrites.

Some dual-boot programs have a special MBR that asks you at startup which operating system you want to use. The fdisk /mbr command erases this program. Dual-boot systems that boot whichever partition is marked Active are not affected by the fdisk /mbr command.

For more information about partitioning, query on the following word in the Microsoft Knowledge Base:
partitioning

The third-party products that are discussed in this article are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, regarding the performance or reliability of these products.

You said that :
Do not use of this command if any of following conditions exist:
* The disk was partitioned using the Storage Dimensions SpeedStor                        utility with its /bootall option.
   * More than four partitions exist.
   * Certain dual-boot programs are in use.

I focus on one of these conditions that tell (more than four partition exist), I execute fdisk and see that I have two partitions(one primary and second extended).
does your purpose indicate  logical drives that exist in extenden partition?
Is there any computer that have more than two partition?

It is possible for a computer to have more than one primary and one secondary partition.  But that's usually only if you have more than one drive, or more than one operating system on your computer.

What is it you're trying to do?

Yes, the extended partition contains your logical drives.  The primary partition and the logical drives are actually what you see as drive letters.  So if you have a primary partition and an extended partition seaperated into two logical drives you would have a C:, D:, and E: drive.

In windows xp, to see how the drives are partitioned, right-click My Computer and choose manage.  In the window that appears go under "Computer Management->Storage->Disk Management" and it will show you how your drives are partitioned.

In Disk Management the partition(s) with the dark blue bar are primary partitions.  The partitions with the light blue bar and the green border around them are logical drives on the extended partition.

hi,
I have a sex drives(c:,d:,e:,f:,g:,H:)on my computer.
can i use of this command(fdisk /mbr) without effecting on these drives?

yes, as long as you don't delete or format the partitions

thanks a lot for your guide.
how  can i make more than two partitions for the end of this question?

I don't understand your question, you mean how do you split the drive into more than two drive letters?

you say that:
"It is possible for a computer to have more than one primary and one secondary partition"
how can i make more than two partition?

yes,i say that
how can partition a hard disk to more than two partition?

i increase my pointe and wait for your answer crash2100

thanks for your guide

what is the version of fdisk /mbr on win xp?
i'm trying to replace my grub boot manager with the one that comes with win xp

You don't use fdisk in windows xp.  It's all done by way of the setup.  You can either do a repair install, or you can use the recovery console and run the fixmbr command:

How to Perform an In-Place Upgrade (Reinstallation) of Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;315341

Computer stops responding with a black screen when you start Windows XP
http://support.microsoft.com/?scid=kb;en-us;314503

FIXMBR / FIXBOOT
http://support.microsoft.com/default.aspx?scid=kb;EN-US;314058

fixmbr device name
Use this command to repair the MBR of the boot partition. In the command syntax, device name is an optional device name that specifies the device that requires a new MBR. Use this command if a virus has damaged the MBR and Windows cannot start.

Warning This command can damage your partition tables if a virus is present or if a hardware problem exists. If you use this command, you may create inaccessible partitions. We recommend that you run antivirus software before you use this command.

You can obtain the device name from the output of the map command. If you do not specify a device name, the MBR of the boot device is repaired, for example:
      fixmbr \device\harddisk2

If the fixmbr command detects an invalid or non-standard partition table signature, fixmbr command prompts you for permission before rewriting the MBR. The fixmbr command is supported only on x86-based computers.


FIXBOOT
     fixboot drive name:
Use this command to write the new Windows boot sector code on the system partition. In the command syntax, drive name is the drive letter where the boot sector will be written. This command fixes damage in the Windows boot sector. This command overrides the default setting, which writes to the system boot partition. The fixboot command is supported only on x86-based computers.