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Partitioning, general information

Summary:
General information about partitioning schemes, the MBR (master boot record) and how to recover/rebuild partition tables. DiskPatch allows you to address problems that are caused by damage to these structures.
The Partition Table

Before you can store data on a hard disk it needs to be partitioned. Consider a partition to be an area of the hard disk (or even the entire disk) that is set aside for use by an operating system such as Windows. Even when you only have a 'c: drive' the disk is still partitioned, in one partition. After a partiton was created it typically needs to be formatted before you can store data on it.

There are several advantages to dividing a hard disk into multiple partitions:

  • Multiple partitions can help you organize your hard disk into several sections. You could for example have one partition to install your operating system in, one for your programs and one to store your data.

  • Dividing a hard disk into multiple partitions can also simplify the process of creating backups and creating so called disk images with for example Symantec's Ghost disk imaging software.

  • Dividing the hard disk in multiple partitions also increases safety; file system corruption will probably affect only one partition while the data in other partitions remains intact.

The Master Boot Record (the first sector on the disk, also known as the MBR) contains the information that describes the primary partitions: the partition table.

 
The active partition, visible and hidden partitions
An active partition is the partition that the computer will try to start the operating system from. Operating systems such as Windows, OS/2, DOS and Linux are usually installed in an active partition. There must be at least one active partition on the disk if that disk should be used to start the computer.

If no active partition is set, or if multiple partitions are set active, the system will fail to boot from the hard disk.

If you have a multiple boot configuration (multiple operating systems installed on one hard disk; this usually means that there is more than one partition, each containing an operating system) it may be desirable to hide primary partitions you are not currently booting from so they are not accessible by the active operating system preventing corruption.
 
The MBR boot loader program
Apart from the primary partition table, the MBR also contains boot code that is required to start your computer. This boot code is the first program code that is executed after the system BIOS finishes its self tests (also known as POST, Power On Self Test).

The main purpose of the boot program code is to locate the active partition.
 
Backing up the MBR and Partition Table(s)
The MBR is a single point of failure! If it is overwritten or becomes corrupted you will lose access to all partitions and your data.

All primary partitions are defined in the MBR's partition table. The extended partition, which is the starting point for a chain of partition tables defining your logical partitions, is also defined in the MBR. If this chain is broken at any point you will lose access to one or more logical partitions.

The MBR and the partition tables are just a few sectors, but they are vital sectors: if they become damaged you will lose access to your data.

Having backups of these vulnerable areas will simplify recovering data that was lost due to damage to the MBR, the partition table(s) and boot sectors.
 
Partition Table Repair and Recovery
A damaged MBR or corrupted partition table will almost always result in mass data loss; access to entire partitions or even the entire hard disk is lost.

Accidental partition deletion will make all data in the deleted partition inaccessible. Recreating the partition with Disk management or Fdisk will only increase the damage.

To recover the data in the lost partitions special data recovery software is required; it should be able to find the lost partitions and reconstruct the partition table(s) while leaving the contents of the partitions intact. Reconstruction of partition tables on the disk itself is referred to as 'in-place' repairs.

Whatever software you select for this task, it must be able to undo the repairs in case the repairs lead to undesired results!

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