  Partition Rescue mini HOWTO
  Jean-Daniel Dodin
  july 27, 2000 - do005722_partition_03

  Whow ! my disk is empty ! My Linux is gone ! If you have or fear to
  have one day or an other such a problem, read this...
  ______________________________________________________________________

  Table of Contents


  1. What's in

  2. What to do right now ?

  3. Legal stuff

  4. What do I must know right now ?

  5. Disks

  6. Partitions

  7. Why is there a problem ?

  8. Solving the problem

     8.1 The simpler case
     8.2 A not so simple case
        8.2.1 By hand
        8.2.2 gpart
     8.3 The rich man case

  9. Authors

  10. Most recent version



  ______________________________________________________________________

  1.  What's in

  This mini-HOWTO addresses only the "lost partition table" problem.
  This can be when :


  ·  you have no more access to your computer, with the "no operating
     system" message,

  ·  you have installed a new system (ie windows) and you see no more
     Linux, and windows take up all the capacity of the disk.

  Here you will learn that if you know the right thing and do it, Linux
  comes usually safe from such things. Windows can, but it's luckier.

  We will first see what you can do before the problem to ease the
  future recovery and what you must do after to recover. There is little
  to do to prevent from erasing a disk, usually this is done by
  automatic windows or Linux-install ill behaved programs or users
  mistakes - nothing can be done to prevent this except care, but you
  are already careful, isn't it ?

  It can also be done by the use of MSDOS/Windows fdisk. avoid it as
  most as you can, but you probably can't.

  I have done this many times, on my computer and on others' guy
  computers and restored Linux most of the time and windows sometimes. I
  wish you luck !

  2.  What to do right now ?

  If you read this by curiosity or just seeking information and you are
  on a running Linux system, do immediately the following :


  ·  open a root terminal or xterm,

  ·  key in "/sbin/fdisk -l" (that last character being l for Lima).

  you will be gratified by a list of all current partitions on all disks
  presents on your computer.


  ·  Write this back on paper (or do "/sbin/fdisk -l | lpr" to print it)
     and save it in a safe place for future use.

  If you are not the system administrator, you should not be concerned
  by the problem and can stop reading this.

  3.  Legal stuff

  This mini-HOWTO is Copyright (c) 2000 by Jean-Daniel Dodin. All rights
  reserved. You may do (almost) whatever you want with it. I don't care.
  Just be sure to keep my name intact. I just hope it becomes useful for
  any person in the Linux community.

  I am not responsible of any damage on any computer as a result of
  anyone reading this HOWTO. If you do any damage, _it is YOUR fault,
  NOT MINE !_ Be careful when partitioning disks, and don't make any
  mistakes, because it can be fatal ! Backup all your important data and
  check that everything you do is correct ! What is described here
  worked on my computer, but it may or may not work on your computer.
  Although it should work for everyone, I can't guarantee anything. This
  is the last warning you get : _BACKUP IMPORTANT DATA !_ Or, to put it
  short: Use at your own risk !

  4.  What do I must know right now ?

  You must know that in case of any major problem with your hard disk,
  you must stop using it at all in write mode, at least the time
  necessary to understand what happens. Information there is very
  volatile...

  If ever, one morning, awaking, you computer say "can't load, no system
  installed", you must not begin reinstalling all the stuff.

  If you have windows installed, I can't promise you can recover your
  data, but it's likely you will recover all your Linux stuff, provided
  it's not located too low in the disk structure. This is because some
  windows viruses erases the very first disk cylinder, whatever is on.
  However I didn't ever experiment such virus and can't say for sure.
  Try recovering anyway.

  You must also know that I give you all this information only for this
  -information purpose. Neither I nor any other people but you can be
  held responsible for any problem your data can have using this info.
  There are too many different systems on the world for anybody being
  able to promise anything. I can only wish you luck and hope you, like
  me, will be happy recovering data.


  5.  Disks

  A hard disk is made of sectors numbered from 0 to the max.

  dmesg gives, for example :

  hdb: ST34321A, 4103MB w/128kB Cache, CHS=523/255/63

  CHS means Cylinders, Heads, Sectors.

  523*255*63=8401995 sectors of 512 bytes, thus the 4103Mbytes. This is
  only a logical map, it's not necessary what is written on the disk
  cover (except for the total size).

  The true size of the sectors is of no interest for us given we don't
  want to modify anything but restore a previous state. For us, the
  default size given by fdisk is alright.

  The size seen by the system is directly dependent of the work of the
  BIOS (Basic Input/Output System - the PC's ROM). The mode of the hard
  disk indicated in the BIOS is essential. On a new disk, it's better to
  use BIOS automatic hard disk recognition and say "yes". Anyway, any
  modification at this level may destroy all the data of the disk, so
  don't play with this without essential reason.

  This is probably what your disk use, so don't be afraid.

  6.  Partitions

  Disk are now huge, 13 Gb are not rare, so that it's not really handy
  to have all this stuff packed in only one part. Only windows do so,
  and if you use Linux, may be it's because you are aware of how
  inefficient the other is.

  So a hard disk is usually cut in some pieces called "partitions" (see
  the "partition mini HOWTO" for details, also read
  /usr/doc/package/util/README.fdisk)

  Let's get a look at (part of) my own print of fdisk -l :

  Disk /dev/hdb: 255 heads, 63 sectors, 523 cylinders Units = cylinders
  of 16065 * 512 bytes

  Device Boot Start End Blocks Id System

  /dev/hdb1 1 153 1228941 83 Linux

  /dev/hdb2 154 166 104422+ 82 Linux swap

  /dev/hdb3 * 167 291 1004062+ 83 Linux

  /dev/hdb4 295 523 1839442+ 5 Extended

  /dev/hdb5 295 422 1028128+ 83 Linux

  /dev/hdb6 423 523 811251 6 FAT16

  This is my second hard disk, tied to guesses and tries (the first is
  too simple to be interesting).

  /dev/hdb is my second ide disk (slave on the primary interface),

  /dev/hdb1 is the first primary partition, running from the first (1)
  block to the block 153.


  There can be four of such primary partitions. If one wants more than
  4, one of them must be an "extended" one (not necessarily the fourth)
  and all other partitions are named "logical" and are located inside
  the extended one. Notice that partition number 5 and partition number
  4 have the same beginning. Number five is logical, number 4 extended.
  Logicals begin always at 5, even if there are not 4 primary ones.

  7.  Why is there a problem ?

  Problem is all the installed Operating Systems must share the disks
  and as at start the bios only scan the first one, there must be a so
  called "partition table" in the very beginning of this disk. This
  partition table is located in the Master Boot Record (MBR) side by
  side with the boot loader.

  Any misuse of the MBR by any of the OS's leads to problems. When
  trying to install any system, yes answer at a question like "automatic
  partitioning ?" is likely to give problems... This is specially true
  with windows, especially with custom windows installation made by
  special makes PC's (when no true "windows" cd is included). But it's
  also true with new "smart" (not so smart !) Linux installation
  programs given with most recent distributions.

  8.  Solving the problem

  Please, beware ! following the explanations given here will lead you
  to turn back to a previous system, loosing all your recent installed
  one, if any !  You must choose...

  8.1.  The simpler case

  All is simple if you have at hand :


  ·  a disk (floppy or cd) able to start Linux by itself with fdisk
     available - most rescue disks of any distribution can do that,

  ·  a paper with the fdisk -l content written down.

  It's enough to


  1. start Linux,

  2. start fdisk /dev/hda (or whatever is the disk to rescue),

  3. use fdisk to delete (d option) all the existing partitions on the
     damaged disk,

  4. use fdisk to create all the primary (1-4) partition mentioned on
     the paper,

  5. give them the appropriate tag (t option) : 82 is for Linux swap, 83
     for Linux main (L gives you the list), 5 is extended and must be
     done before creating logical partitions.

  6. create any logical partition.

  fdisk is a small and very smart programs. There are many other makes
  of fdisk, but I always prefer the bare bone one (I speak of Linux
  ones, of course, not the others...).

  Be aware that fdisk doesn't write anything to disk before you hit w
  and return. In case you fear a mistake, hit q (quit) or Ctrl C (\^{
  }C) to quit safe.

  When your new partition table is written, start your Linux. Chance is
  you can't do that as usual : lilo can have been damaged also and you
  will need a boot floppy or booting from a cd (choose the option
  "booting the installed partition").

  If you use to boot with lilo, as soon as you are logged in as root,
  key in "lilo" and hit return to reinstall you favourite boot loader.

  Your Linux should be all here, test it. Try also to start windows if
  applicable.  If you can't, there is a (very little) chance you can
  read your data from Linux, may be with a raw sector by sector read. If
  you can identify the disk sectors you data is on, using dd can copy
  them on a file. This is wise for text only.  This recovery is NOT in
  the scope of this mini-HOWTO.

  8.2.  A not so simple case

  8.2.1.  By hand

  This is when the previous case can't be used, for lack of fdisk paper
  or if it won't run for use of an out of date one.

  First, be aware that as soon as you don't write to the disk (except
  with fdisk), you can't erase your data, so that you can use a block by
  block try.  That is you need to know the beginning of the partition to
  start it. If, say a 153 don't fit, try a 154, and so on.

  This can be tiresome, but if you remember approximately the size of
  the Linux partition, there is a chance to win.

  8.2.2.  gpart

  But there is a better way if you can still access the net or have
  "gpart" at hand.

  "gpart - guess PC-type hard disk partitions" is the first line of the
  man page of gpart (man gpart).

  "gpart tries to guess which partitions are on a hard disk. If the
  primary partition table has been lost, overwritten or destroyed the
  partitions still exist on the disk but the operating system cannot
  access them.". This is exactly what we need.

  gpart is still beta, but is already a very good tool.

  The problem is the following : the first block of any partition is
  marked.  But it's never "unmarked" if not overwritten. So many "first
  partition block" are existing on an old disk and gpart tries to do
  it's best guessing what is the good one. In fact it's not too
  difficult to try, nothing is written on the disk by gpart.

  Here is the result of gpart on the previously seen disk hdb :

  root@charles:/home/jdd > gpart /dev/hdb

  Begin scan...

  Possible partition(Linux ext2), size(1200Mb), offset(0Mb)

  Possible partition(Windows NTFS), size(1200Mb), offset(1200Mb)

  Possible partition(Linux ext2), size(1004Mb), offset(2402Mb)

  Possible partition(Windows NTFS), size(1600Mb), offset(4102Mb)


  End scan.

  Checking partitions...

  * Warning: partition(OS/2 HPFS, NTFS, QNX or Advanced UNIX) ends
  beyond disk end .

  Partition(Linux ext2 filesystem): primary

  Partition(OS/2 HPFS, NTFS, QNX or Advanced UNIX): primary

  Partition(Linux ext2 filesystem): primary

  Partition(OS/2 HPFS, NTFS, QNX or Advanced UNIX): invalid primary

  Ok.

  Guessed primary partition table:

  Primary partition(1)

  type: 131(0x83)(Linux ext2 filesystem)

  size: 1200mb #s(2457880) s(63-2457942)

  chs: (0/1/1)-(152/254/61)d (0/1/1)-(152/254/61)r

  Primary partition(2)

  type: 007(0x07)(OS/2 HPFS, NTFS, QNX or Advanced UNIX)

  size: 1200mb #s(2457880) s(2457944-4915823)

  chs: (152/254/63)-(305/253/60)d (152/254/63)-(305/253/60)r

  Primary partition(3)

  type: 131(0x83)(Linux ext2 filesystem)

  size: 1004mb #s(2056256) s(4919781-6976036)

  chs: (306/61/49)-(434/60/47)d (306/61/49)-(434/60/47)r

  Primary partition(4)

  type: 000(0x00)(unused) size: 0mb #s(0) s(0-0) chs: (0/0/0)-(0/0/0)d
  (0/0/0)-(0/0/0)r

  As you see, primary partition can be recovered, but for extended ones
  it's still to be done.

  Dos partitions are labelled "windows NTFS" because they were created
  while trying to install Windows 2000 (a very awful experience !). The
  "invalid" one is, in fact the extended partition.

  With this, one can use fdisk and try re-creating the partition table
  (remember, this is risk-free given the original one is already lost).

  8.3.  The rich man case

  Partition Magic is a commercial product, not so cheap given the little
  use one can have (approx a hundred bucks in France) but with a very
  high reputation all around there. However I never use it and will not
  rate it. It's said to be able to do anything with partitions,
  including restoring them.

  Original Ralf partition-rescue mini HOWTO was essentially based around
  the use of Partition Magic, so I presume it's a very good solution if
  you have valuable data on your Linux partition and little Linux
  capability. However there are now very recent makes of Partition Magic
  and I think it's better for you to read the manual.

  9.  Authors

  The author of this HOWTO is Jean-Daniel Dodin. I can be joined at
  jdanield@dodin.net or at jdanield@linux-france.org.

  My web site is at http://www.dodin.net. Linux pages, who are of most
  interest for you are found at www.linux-france.fr/article/jdanield.

  I want to thank Rolf Klausen (E-mail: rolfk@romsdal.vgs.no
  http://www2.romsdal.vgs.no/~rolfk/) who write the previous partition-
  rescue mini HOWTO even if I rewrite it almost entirely, he had first
  the good idea.

  Every other member of the Linux community and everybody who supports
  Linux and writes documentation and programs for Linux and all the
  authors of the LDP and virtually any person involved in anything which
  has to do with Linux.  Particularly Linus B. Torvalds - he is _The
  King_ !!!

  I want also to thank Michail Brzitwa <mb@ichabod.han.de> for writing
  gpart !

  10.  Most recent version

  The most recent version of this mini-HOWTO shall be found on my Linux
  web site, at www.linux-france/article/jdanield/



