FDISK /MBR Rewrites the Master Boot Record

ID: Q69013

The information in this article applies to:

SUMMARY

The MS-DOS Fdisk utility usually updates the master boot record 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 drive 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). At the beginning of this sector of the hard disk is a small program. At the end of this sector is where the partition information, or partition table, is stored. 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 what is written to the disk by FDISK /MBR and is usually called the master boot record. During normal operation, Fdisk writes this program to the disk only if there is no master boot record.

Why is the MBR Changed During Setup?

During 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 following: 

   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 fails to 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: 

   C:\>FDISK /MBR

WARNINGS

This option should NOT be used if any of the following conditions exist:

The Storage Dimensions SpeedStor utility with the /BOOTALL option redefines the drive's physical parameters (cylinder, head, and sector). /BOOTALL stores information on 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 FDISK /MBR overwrites.

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

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

   partitioning

SpeedStor is manufactured by Storage Dimensions, a vendor independent of Microsoft; we make no warranty, implied or otherwise, regarding this product's performance or reliability.


REFERENCES

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

Additional query words: 6.22 5.00 5.00a 3rdparty third-party third party 6.00
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Keywords          : kbtool msdos win95 win98 
Version           : 
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Issue type        : kbinfo

Last Reviewed: January 12, 1999