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.

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.

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.

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 information in this article applies to:

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