The following article is the second in a series that helps solve many of the common boot problems with hard drives.

The article explains how to reconstruct the 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 the error message "MISSING OPERATING SYSTEM."

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

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

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

Rebuilding the MBR

Using the Fixed Disk Editor in Micro-Scope, locate and read Cylinder 0, Head 0, Sector 1. This should be the location of the corrupt Master Boot Record. Press "M" to modify the sector and enter 0’s for the entire sector, effectively wiping all the data from this sector. Press ESC, then "W" to write the data, and "Y" to confirm the write operation. Then exit the Fixed Disk Editor and select the 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 the reconstruction of the VBR.

Rebuilding the VBR

There is a simple method for rebuilding the VBR. Copy the data from a like fixed disk or a similar-sized fixed disk with the identical operating system (the versions must be the same.) There are two approaches for obtaining this data. The first is to install the second drive in the system, reboot Micro-Scope, and copy the sectors directly from the "healthy" drive to the drive being reconstructed. The second method is to copy the sectors from the "healthy" drive to a floppy diskette and transfer that diskette to the machine containing the drive being reconstructed. The data can be copied to any sector on the floppy disk. Just note where this data is stored on the floppy disk so that it may be easily retrieved when copying it back to the drive being reconstructed. In either case, the VBR is located at Cylinder 0, Head 1, Sector 1 on the "healthy" drive. Copy this data to the same location on the damaged drive. Exit the Fixed Disk Editor.

The drive now has a clean MBR and VBR, and with a few modifications to the values stored in these data areas, the drive will be fully reconstructed.

Access the VBR in the Partition Editor. Some of the values in this area will remain the same. These are: Bytes Per Sector, Reserved Sectors, Number of Copies of FAT, Max # of Root Dir. Entries, Media Description Byte, and Volume Boot Signature. Sectors Per Cluster may or may not need to be changed. Put the cursor over this value and press F1 for a formula that will help determine the proper value. Use this same F1 procedure for determining the Total Number of Sectors. This value should be the same as the value for Number of Sectors in the partition table.

The Sectors per FAT value equals the number of sectors from the beginning of the first FAT to the beginning of the second FAT (as described in the first article in this series.) The values for Sectors per Track and Number of Hidden Sectors should be identical to the values in the MBR. The Number of Heads is either the same as the value in the MBR or the same as the value of the Number of Heads in the partition table plus one (refer to the first article in the series for more information on this topic.) After completing this information, the drive should either be bootable or accessible via a bootable diskette. If this is not the case, e-mail the Technical Support department for further verification of the problem.

Reconstructing Multiple Partition Volumes

In some cases, the drive may have been partitioned into smaller, logical partitions. If this is the case, the chance of this data being corrupt is slim, and correct Head and Sector values are readily available from these areas. To find the second partition on the drive, use the Fixed Disk Editor in Micro-Scope to view the sector containing the "55 AA" string. This sector should appear after the first VBR, so don’t confuse the two. The second MBR should start on Head 0, Sector 1 of a given cylinder. Accordingly, the second VBR will appear on Head 1, Sector 1, and the FAT on Head 1, Sector 2, etc. By adjusting the first MBR in Micro-Scope so that the value for Starting Cylinder, Head and Sector is equal to the location of the second MBR, the VBR is now accessible through the Micro-Scope partition editor. Use these values to reconstruct the second MBR. The only value that will need recalculating is the Start Absolute Sector in the second MBR.

As outlined in the first article in the series, the Start Absolute Sector is the same as the value for Ending Sector. This is not the case with a second partition. The Start Absolute Sector is the number of sectors prior to the VBR. The formula for the Start Absolute Sector is as follows: (Number of Sectors) + (the Start Absolute Sector of the previous partition) + (Sectors per Track). This formula will work for any additional logical partitions.