The article explains how to reconstruct Volume Boot Record and will touch briefly on multiple-partition drives. This information will help solve some very common problems encountered in a computer service department. This article is designed to work with a system that has a single drive using a FAT 16 structure (all DOS and older Windows systems, some Windows NT), and no drive overlay.

A common problem with hard drives is error message "MISSING OPERATING SYSTEM."

Typically what this means is that critical data is missing or damaged in two possible locations. The first is Master Boot Record (MBR). This data area contains error messages and pointer information that specifies location of second critical data area, Volume Boot Record (VBR). The VBR contains operating system specific data for format of that particular partition on drive. If either of these two critical data areas is damaged, by a virus, controller failure, etc., then drive will not be accessible through operating system.

The normal response to this error message is to boot to a floppy disk and to re-partition drive (probably using FDISK) and thereby totally removing any possibility for recovery of data on that drive, except by dedicated data recovery software or a data recovery facility. If applicable, following technique will help save time, money, and frustration.

The first step to data recovery (covered in great detail in first article of this series) is to verify location of existing partition information. For purposes of this article, it will be assumed that FAT structure and root directory are intact, there is only one partition, and that only MBR and VBR were damaged. Using Micro-Scope, enter Fixed Disk Editor menu and select fixed disk that is being repaired. Using Find menu option, locate all sectors that contain string "55 AA" (see Step Four of first article in this series), and then verify that FAT structures are intact (see Step Seven). Note on a scratch paper values (in Cylinder, Head, Sector format) where this string appears. These values will be used later in reconstruction process.

Rebuilding MBR

Using Fixed Disk Editor in Micro-Scope, locate and read Cylinder 0, Head 0, Sector 1. This should be location of corrupt Master Boot Record. Press "M" to modify sector and enter 0's for entire sector, effectively wiping all data from this sector. Press ESC, then "W" to write data, and "Y" to confirm write operation. Then exit Fixed Disk Editor and select Rebuild Master Boot Record menu option. Press "Y" to confirm this operation. The fixed drive now has a completely corruption-free MBR available for use during reconstruction of VBR.

Rebuilding VBR

It is extremely important not to make or write any changes to data on hard drive without first verifying hard drive configuration. Therefore, first priority when unable to access information on a hard drive is to verify all of configuration information dealing with suspect hard drive.

Partition Parameter Value or Equation Partition Status Bootable Starting Head 1 Starting Sector 1 Starting Cylinder 0 Partition Type Bigdos (drives greater than 30 MB), otherwise DOS 12 Ending Head Total Number of Heads -1 Ending Sector Sectors per Track displayed in Volume Boot Sector Ending Cylinder Number of Cylinders set in CMOS - 2 Total # of Sectors Number of Sectors displayed in Volume Boot Sector Start Absolute Sector Number of Sectors per Track in Volume Boot Sector Boot Signature 55AA

Step One: Make sure that Micro-Scope detects hard drive

In Micro-Scope, System Configuration, Compare Settings - check to make sure that there is not an asterisk beside number of hard drives detected value. If there is an asterisk, either CMOS is set incorrectly, or there is an electronic problem (controller, cable, drive).

Step Two: Compare BIOS parameters against drive partition information.

In Micro-Scope, System Configuration, System Information - note information displayed for hard drive, specifically parameters for drive in question. Compare these parameters to parameters in MBR display under Micro-Scope, System Configuration, Partition Display using following formula:

System Information Master Boot Record Cylinders Ending Cylinder + 2 Heads Ending Head + 1 Sectors Sectors Per Track

If information does not match, either partition information is corrupt, or CMOS setup or controller BIOS setup (if one exists) is incorrect, or there is a problem communicating with drive.

Step Three: Check drive functionality

In Micro-Scope, Diagnostics, Fixed Disk Tests - check information in Fixed Disk Selected window. Make sure that proper drive type, model, and native parameters are showing in this window. If any of information is incorrect, there is an electronic problem. Check cable, controller and drive, re-seating all connections, and repeat above procedure. After correcting electronic problem, perform a read test on first 10 cylinders of drive. If any errors occur, then this is most likely cause of drive failure. Perform a read of entire drive to determine if errors are electronic in nature, or physical in nature. Electronic problems will result in errors displayed not always being same type or in same location on drive. If problem is electronic, replace cable, controller, and finally drive electronics and repeat read test until no errors occur. If error is physical, use an INT 13 type editor (such as Norton Advanced Editor), to block entire drive and write information to a daisy chained drive set to same parameters as faulty drive.

The next step, after drive passes read test on first 10 cylinders, is to check original partition setup of drive.

Step Four: Check for physical location of master boot sector and volume boot sector.