Partition recovery

What is a partition - a simple overview

The Master Boot Record (MBR) and optionally several Extended Partition Pointers (EPPs) are used to hold information about how many volumes there are on the physical disk, and where they are located. MBR and EPPs occupy 512 bytes (one sector) each. MBR is located in a sector 0 of the physical disk and contains some machine code responsible for the boot process. It also contains up to four entries describing a partition. Each of those may be unused, may point directly to the volume, or point to the EPP block. Each EPP block contains a pointer to the volume and an optional pointer to the next EPP block in chain. This way, one can split a single physical disk into more than four logical volumes. A partition defined in MBR is called Primary Partition, as opposed to the partition defined in one of the extended partition chain EPPs, which is called Logical Drive.

These tables (MBR and EPPs) are collectively referred to as Partition Table. The set of the EPP blocks (and sometimes volumes contained therein) is called Extended Partition.

If the Windows 2000/XP or later "Dynamic Disks" are used, the partition table is not used (except for a boot process and one compatibility entry). LDM database  is used instead.

Some typical partitioning layouts are illustrated below:

(1) Features primary volumes only, with no extended partitions present.
(2) Features one primary volume and one logical drive with the simple (non-chained) extended partition record
(3) Presents the example containing one primary volume and two extended volumes (extended partition records are chained)

There are some limits: the MBR can only contain up to four records in total (counting both primary partitions and a pointer to EPR, if any). In theory, the EPR may contain four entries as well (thus making extended partition chain forks possible) but in practice this is never used. Also, the logical drives in the extended partition cannot be made "active" (bootable).

Why partition recovery is sometimes needed - typical failure modes.

Partition tables become corrupt for various (often obscure) reasons, causing various symptoms, including but not limited to the following most common:

• Some volumes just disappear. Disk Manager may show either unallocated space or something weird where the volume(s) are supposed to be.

• System may refuse to boot up with messages similar to "Bad or missing partition table", "Error loading operating system", or without any messages at all (in this case double check cabling and SCSI termination, BIOS settings, and that BIOS does successfully detect the drive).

• "Phantom" volumes or free space areas may appear in the Disk Manager (i.e. the ones you did not create). The total storage capacity (calculated by summing up all volume and free space area sizes) may be exceed the capacity of the physical disk. This indicates that some volumes overlap with each other, a particularly dangerous situation because writes to one volume end up damaging the other one. Note that Disk Manager GUI has no way to indicate such an overlap, you need to perform the calculations yourself.

• In rare cases Windows blue screen STOP: INACCESSIBLE_BOOT_DEVICE is caused by the damaged partition table, most likely reasons for this error being a RAID drivers and/or BIOS/cabling issues.

Typical partition damage profiles are illustrated below.

Note: red vertical lines indicate inaccessible data.

(4) illustrates the most simple situation caused by an operator error - the deletion of a wrong volume. The data is still intact and in place, albeit inaccessible. Since the reference to the volume is deleted, there is no way for the operating system to reach that data.
(5) is the example of a significant localized damage, similar to that caused by some viruses. This is the worst case scenario: all the partition table entries are either damaged (MBR) or inaccessible (no route to locate EPPs). On top of that, the boot sector of a primary volume is damaged and in case of FAT32 file system, the backup boot sector is also gone (since it is stored close to the primary one).
(6) illustrates partition chain corruption. Note that both logical drives are lost once the first link has been broken.

Understanding partition recovery

The key to successful partition recovery is knowing the sizes and locations of the missing volume(s). The simplest situation is if the disk was partitioned as a single logical drive. In this case it is efficient to assume that volume occupies the whole physical drive and let the filesystem recovery algorithm do the rest. The "slack" space before and after the volume is typically small and does not create any significant distortion. Partition recovery becomes more complicated when multiple volumes are involved, because the damage to MBR or somewhere early in the extended partition chain makes multiple volumes disappear at once. The difficult part is then to define the boundaries between the volumes. To solve the problem, utilize following features of the on-disk layout to the full extent:

• The boot sectors (and their backup copies if any, see below) may be still be intact on the disk. ZAR can search for and identify any such remains. Manual attribution of the found objects to their corresponding volumes is still required, but the disk scan results are in most cases helpful.

• The volumes are placed close to each other. "Slack" (unused) space between them is typically about 64 or 128 sectors (32KB and 64KB respectively). For partition recovery purposes, it is in most cases safe to disregard this slack space and treat the allocation as contiguous. This provides a last-resort information: one can derive the location of the volume by simply summing up the sizes of the volumes before it.

Take a special note that FAT series filesystems (FAT16 or FAT32) place their metadata very close to the start of the volume. So, the filesystem recovery process is much more sensitive to lower (start) boundary of the volume than to the upper (high) boundary. Keep this in mind when manually defining the areas for the filesystem recovery.

False positives while scanning for missing partitions

In-place repair possibilities