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, or CD-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), 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. You may be able to fix this by using the Recovery Console in Windows XP, your Emergency Repair Disk sets in Windows 2000 or NT, your Recovery Disks in Windows 95 or 98, some specialty utilities, or, in some cases, Recovery Disk sets built from a virus protection program, such as Symantec/Norton AntiVirus.

The most commonly used utility for repairing problems with the MBR is the fixmbr command (XP and 2000) or the fdisk /mbr command (95, 98, Me, and NT).

Some of this information comes from article 69013 in the Microsoft knowledge base.

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Also see:

• Can I access an NTFS partition without booting Windows 2000 or XP?
• In Windows, if BootMagic is no longer installed on my computer, how do I remove the error message "Error finding BTMagic.img" at startup?