Boot sector

Master Boot Record (MBR) and partition recovery

Probably the most important sector on any disk is the boot sector, or sector 0. When the sector has become corrupted, or failed, most PCs will no longer recognize the disk, and will not boot. In these cases, partition recovery is required

CnW Recovery has developed tools that will analyse the disk and enable the boot sector to be reconstructed so that data may then be recovered. It is always important to know why the sector has failed.  If it was due to say a virus, or operator error, then reconstructing the sector on the original disk can work. If it was due to a failing hard disk, any on disk reconstruction may only be temporary.  CnW will always advise if possible starting with a new hard disk.  Currently, the cost of a new drive is much less than the value of the data, and normally less than the cost of recovering data if the disk were to fail again.

The boot sector has two very important sections of information.  The first section is a short piece of code to set the computer loading the operating system. Towards the end of the sector, (byte 1BEH) is is a table containing details of up to 4 separate partitions on the disk drive.  This table is described later on, but first, what is a partition? The final two bytes of the sector always contain the values 55H  AAH which is a signature and check values

 What is a partition?

A physical disk drive is logically just a series of sectors, numbered from 0 to ‘xxxx’  where ‘xxxx’ can be a very large number, currently less than 4G.  This means that with standard software tools, and 512 byte sector, a disk has a maximum size of 2TB. A large disk can become very unwieldy and so very often large disks are partitioned in smaller logical disks.  Each logical disk looks like a stand alone disk to application programs. This approach also means that some users can separate their data, with maybe programs on one disk, and data on another disk.  Each disk is then fairly resilient against viruses and user errors on another logical disk, but all logical disks still depend on the same boot sector.

A partition is therefore is just a logical, rather than physical, allocation of sectors to a disk. The original limit was 4 partitions per disk, but with extended partitions, the number may be much higher - each partition is linked to the previous partition

Structure of the partition table

The partition table is 16 bytes (10H) for each table, and starts at location 01BE(hex) in the boot sector. The elements of the table is as below.

Partition Recovery

CnW will perform partition recovery on any disk where the partition table has been corrupted, or the boot sector has failed.  Occasionally, users can try and reformat the a disk, or change partition details, and if an error occurs the disk may become unreadable. By analysing any remaining boot sector information, and header blocks for partitions, a disk may be reconstructed. Where there are logical conflicts, CnW and override program defaults to configure the drive as required.  It would also be able to recover data from two conflicting partition layouts, thus get data that was written to the disk before a partition was changed.

 

CnW Recovery  Lewes  East Sussex  UK

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