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...
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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
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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/