What is a Master Boot Record (MBR)?

At the end of the ROM BIOS bootstrap routine (which occurs when your PC first boots up), the BIOS reads and executes the first physical sector of the first boot device (floppy, hard disk, CD-ROM, or DVD-ROM) on the system. This first sector is called the Master Boot Record (also known as the partition table, or master boot block). At the start of this sector is a small program. This program uses the partition information (or partition table) stored at the end of the sector to determine which partition is bootable (usually the first primary DOS partition), and then attempts to boot from it.

The values in the partition table (contained in the MBR) depend directly on the size of the physical disk and on the logical partitioning on that disk. The DOS Boot Record (DBR) holds the Boot Parameter Block (BPB), which contains the logical mapping information for that particular partition. The values in the Boot Parameter Block are absolutely dependent on the size of the partition and the type of file system.

These two sectors contain the information necessary to locate and identify the file system used to access the data on the drive. If either of these are damaged, the data becomes inaccessible, even though there may be no damage to the data or the file system itself.

Windows Vista uses a program called Bootrec.exe to recover or repair the MBR. UITS recommends running the "Startup Repair" feature from System Recovery Options first, but if that fails to address the issue, you can run Bootrec.exe from the installation disk, if available. Refer to Microsoft article 927392 for instructions.

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In Windows XP, the most commonly used utility for repairing problems with the MBR is the fixmbr command.

You may be able to repair the problem by using the Recovery Console; see In Windows XP, how do I use the Recovery Console? In some cases, you can also use recovery disk sets built from a virus protection program, such as Symantec/Norton AntiVirus.

Some of this information comes from Microsoft article 69013.

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