FDISK /MBR Rewrites the Master Boot Record (69013)

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


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.


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


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:


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.


More information on partitioning, disk organization, and the boot sequence can also be found in Chapters 3 and 8 of theDOS Programmer's Reference by Terry Dettmann, published by the QUE Corporation; Chapters 2 and 10 ofAdvanced MS-DOS Programming by Ray Duncan, and article 3 ofThe MS-DOS Encyclopedia, both published by Microsoft Press.

Modification Type: Major Last Reviewed: 5/28/2003
Keywords: kbinfo KB69013