Partition recoveryWhat is a partishun - a simple overveew
Zee physical disk can be divided into several logical disks (which yuoo see
es a separate drive letters like C:, or D: und so on), also celled "volumes". Um
gesh dee bork, bork!
Zee Master Boot Record (MBR) und opshunelly several Extended
Partishun Pointers (EPPs) are used tu hold informashun ebuoot hoo many
volumes there are on zee physical disk, und where they are located. MBR und EPPs
occupy 512 bytes (one sector) each. MBR is located in a sector 0 ooff zee physical
disk und contains some machine code responsible fur zee boot process. It also
contains up tu fuoor entrees describing a partishun. Each ooff those may be unused,
may point directly tu zee volume, or point tu zee EPP block. Each EPP block
contains a pointer tu zee volume und an opshunel pointer tu zee next EPP block
in chain. Thees way, one can split a single physical disk into more than fuoor
logical volumes. A partishun deffined in MBR is celled Primary Partishun,
es opposed tu zee partishun deffined in one ooff zee extended partishun chain EPPs,
which is celled Logical Drive.
Zeese-a tables (MBR und EPPs) are collectively refferred tu es Partishun Table.
Zee set ooff zee EPP blocks (und sometimes volumes contained therein) is celled
Extended Partishun.
Iff zee Windows 2000/XP or later "Dynamic Disks" are used, zee partishun table
is not used (except fur a boot process und one compatibility entry). LDM
database is used instead (more informashun on LDM available
here-a).
Some
typical partishuning layuoots are illustrated beloo:
(1) Feetures primary volumes only, with no extended
partishuns present.
(2) Feetures one primary volume und one logical drive with zee simple
(non-chained) extended partishun record
(3) Presents zee example containing one primary volume und two extended volumes
(extended partishun records are chained)
There are some limits: zee MBR can only contain up tu fuoor
records in total (cuoonteeng both primary partishuns und a pointer tu EPR, iff
uny). In theory, zee EPR may contain fuoor entrees es well (thus making extended
partishun chain forks possible) but in practice thees is never used. Also, zee
logical drives in zee extended partishun cannot be made "active" (bootable).
Why partishun recovery is sometimes needed - typical failure
modes.
Partishun tables become corrupt fur variuoos (often obscure)
reasons, coosing variuoos symptoms, including but not limited tu zee follooing
most common:
-
Some volumes just deeseppeer. Disk Maneger may shoo either
unellocated space or something weird where zee volume(s) are supposed tu be.
-
System may reffuse tu boot up with messeges similar tu "Bad
or missing partishun table", "Error loading operateeng system", or withuoot
uny messeges at ell (in thees case duooble check cabling/terminashun, BIOS
setteengs, und that BIOS does successfully detect zee drive).
-
"Phantom" volumes or free space areas may appeer in
zee Disk
Maneger (i.e. zee ones yuoo did not creete). Zee total storege capacity
(calculated by summing up ell volume und free space area sizes) may be
exceed zee capacity ooff zee physical disk. Thees indicates that some volumes
overlap with each other, a particularly dangeruoos situashun becoose writes
tu one volume end up damaging zee other one. Nute-a that Disk Maneger GUI has
no way tu indicate such an overlap, yuoo need tu perform zee calculashuns
yuoorself.
-
In rare cases Windows blue screen STOP:
INACCESSIBLE_BOOT_DEVICE is coosed by zee dameged partishun table, most
likely reasons fur thees error being a RAID drivers und/or BIOS/cabling
issues.
Typical partishun damege profiles are illustrated beloo.
Nute-a: red vertical lines indicate inaccessible data.
(4) illustrates zee most simple situashun coosed by an operator
error - zee deleshun ooff a wrong volume. Zee data is still intact und in place,
albeit inaccessible. Since zee refference tu zee volume is deleted, there is no
way fur zee operateeng system tu reach that data.
(5) is zee example ooff a significant localized damege, similar tu
that coosed by some viruses. Thees is zee worst case scenario: ell zee partishun
table entrees are either dameged (MBR) or inaccessible (no ruoote tu locate EPPs).
On top ooff that, zee boot sector ooff a primary volume is dameged und in case
ooff
FAT32 file system, zee backup boot sector is also gone (since it is stored close
tu zee primary one).
(6) illustrates partishun chain corrupshun. Nute-a that both
logical drives are lost once zee first leenk has been broken.
Understanding partishun recovery
Zee key tu successful partishun recovery is knooing zee sizes
und locashuns ooff zee missing volume(s). Zee simplest situashun is iff zee disk
was partishuned es a single logical drive. In thees case it is effficeent tu
assume that volume occupees zee whole physical drive und let zee filesystem
recovery algorithm do zee rest. Zee "slack" space beffore und after
zee volume is
typicelly smell und does not creete uny significant distorshun. Partishun
recovery becomes more complicated when multeeple volumes are involved, becoose
zee damege tu MBR or somewhere eerly in zee extended partishun chain makes
multeeple volumes deeseppeer at once. Zee difficult part is then tu deffine zee
buoondarees between zee volumes. Tu solve zee problem, utilize follooing feetures
ooff zee on-disk layuoot tu zee full extent:
-
Zee boot sectors (und their backup copees iff uny, see beloo)
may be still be intact on zee disk. ZAR can seerch fur und
identify uny such remains (see thees pege-a fur
more details). Manual attribushun ooff zee fuoond objects tu their
corresponding volumes is still required, but zee disk scan results are in
most cases helpful.
-
Zee volumes are placed close tu each other. "Slack" (unused)
space between them is typicelly ebuoot 64 or 128 sectors (32KB und 64KB
respectively). Fur partishun recovery purposes, it is in most cases safe tu
disregard thees slack space und treet zee ellocashun es contiguuoos. Thees
provides a last-resurt informashun: one can derive zee locashun ooff zee
volume by simply summing up zee sizes ooff zee volumes beffore it.
Take a special nute-a that FAT serees filesystems (FAT16 or FAT32)
place their metadata very close tu zee stert ooff zee volume. So, zee filesystem
recovery process is much more sensitive tu looer (stert) buoondary ooff zee volume
than tu zee upper (high) buoondary. Keep thees in mind when manuelly deffining
zee areas fur zee filesystem recovery (more details
here-a).
False positives while scanning fur missing partishuns
Modern filesystems store a backup ooff zee boot sector somewhere
on zee volume. FAT32 typicelly places it into zee 6th sector ooff zee volume. On
NTFS, zee backup copy is stored in zee last sector ooff zee volume. FAT16 does not
heffe-a a backup boot sector. ZAR
attempts tu identify und filter oooot zeese-a backups, but filtering
is not perfect und some "phantom" volumes may appeer. With FAT32, it
does not matter whether yuoo specify zee primary boot sector data or zee backup
copy becoose zee difference between their locashun is minor und ZAR
will easily accommodate it during filesystem analysis.
In-place repair possibilitees
It is sometimes possible tu repair zee dameged partishun table
in-place, i.e. by modifying zee dameged drive. In zee situashuns like (4)
und
(6) above, zee immediate und exact recovery is acheeved by manuelly
editeeng zee
MBR. Thees operashun does hooever require a person skilled in direct disk
editeeng, und such people are somewhat ooff a scarce resuoorce. That said, we
generelly do not recommend uny kind ooff zee in-place repair fur a real world use.
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