Boot Sectors

Boot sector is a sector on a hard drive or data storage device that contains code for booting a program stored on some other part of the storage media.  The computer BIOS can be [...]]]> The Fixboot is a Recovery Console command that writes a partition boot sector to a system partition that you can specify.

Boot Sectors

Boot sector is a sector on a hard drive or data storage device that contains code for booting a program stored on some other part of the storage media.  The computer BIOS can be set to select any of several boot devices and then copies the first sector from the boot device to address location 0×7C00.

Different kinds of boot sectors can be encountered on computer hard disks and other storage devices.  A master boot record (MBR) is the first sector of a data storage device that has been partitioned. The MBR sector contains code to locate the active partition and start its Volume Boot Record.

Volume Boot Record (VBR)

A volume boot record is the first sector of a data storage device that has not been partitioned.  It contains code to load and start an operating system installed on that device or within that partition.

Valid boot sectors have a two-byte hex word 0xAA55, called the boot sector signature and must exist at the end of the sector.  If the signature is not present either the BIOS or MBR code will report an error message and halt any OS startup process.

The computer BIOS does not distinguish between VBR and MBR. The firmware will only load and runs the first sector of the storage device. floppy devices or USB flash drives will use a VBR to boot. Hard drives use an MBR. The MBR has the partitioning information and will start the VBR which will load the operating system.

Boot Sector Viruses

Since the code in the boot sector is first executed automatically the boot sectors have been targets for computer viruses. As a warning computer BIOS now have included a feature to prevent writing over the boot sectors without user permission.

FIXMBR

FIXMBR is a Recovery Console command that writes a new MBR to the first sector of a hard drive.  This command can be used to correct a corrupted or infected MBR.  The fixboot command write the partition boot sector.

The FIXMBR website has some additional articles you might find useful.