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Solving the problem

3. 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...

3.1. 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 and fdisk -u -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, c is Windows Fat 32 and f Windows extended when 6 is Windows Fat 16.

  6. create any logical partition.

On my SuSE installation and anytime I had to do this for other peoples, this gives a good result.

However I was said that some fdisk may cut partitions on a sector basis, not cylinder. So the fdisk -u -l version of the paper.

For using the fdisk -u -l listing one must start fdisk -u :-). On my opinion, using sector limit is a very bad idea, but it may have a real use I'm not aware of. Problem is that with cylinder limit it's easy to guess even if you don't have paper. With sector one, there a many more guesses...

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 favorite 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.

3.2. A not so simple case

3.2.1. 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.

3.2.2. Linux own info and other hacks Kernel

If you just destroyed your own partition table, but have not rebooted Linux: Don't reboot! You can still retrieve the partition information stored in the Kernel:

cat /proc/partitions gives

      major minor  #blocks  name
          3     0   19535040 hda
          3     1    2096451 hda1
          3     2    4980150 hda2
          3     3          1 hda3      <----- this marks an extended partition
          3     5    4980118 hda5
          3     6    4972086 hda6 hdparm

      hdparm -g /dev/hda1
      /dev/hda1: geometry     = 2432/255/63, sectors = 4192902, start = 63

You'll need to do a few unit conversions. "#blocks" are usually 1K in length. "Sectors" are disk sectors, often 512 bytes. But usually the disk partitioning tools work in units of cylinders (here 255*63=16065 sectors). Using this information you can build a new partition table. I know the start of the partition, but not the end.

If you know the start of a Linux partition, but not the end, you can still mount it, and learn about the structure. Set the partition table start correctly, and set the end to something very large. See if you guessed correctly with:

e2fsck -n /dev/hd?? 

You can even mount the partition and check the size:

mount -r /dev/hd?? /mnt
df -T

This won't directly tell you where the next partition starts, because of rounding issues. But it can help you get close. Be sure to use the "-n" and "-r" flags to treat the system as read-only!!! Other places partition information is stored

Some distributions record partition information in a file. Of course you probably won't be able to get to this file when you need it. But just in case:

SuSE: /var/lib/YaST/install.inf

(if you are aware of others, please email the maintainer of this document)

3.2.3. gpart

But there is a better way if you can still access the net or have “gpart” at hand. gpart is available in most distribution, by freshmeat.net or directly at http://www.stud.uni-hannover.de/user/76201/gpart.

“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 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 
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 labeled “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).

gpart is updated on a weekly basis :-) and so new makes may be more powerful than I know. Recovering partitions inside an extended partition

Extended partition information is scattered on the disk, not stored with the primary partition. To recover these often requires more works. The process is:

  1. Scan for the start of the first partition (using gpart's -k option);

  2. Create a temporary primary partition entry with the true start position and a fake end position (this may drive you to delete an actual primary partition if no one is available - this is risk free if you don't reuse the sectors of the deleted partition);

  3. Use “e2fsk -n”, “mount -r”, and “df” to determine the true end point. Write this value down (warning:read the man page for each program mentioned, and use the read-only options, you do not want to write to your disk until all partitions are in the correct place);

  4. Repeat this process for each partition to be recovered;

  5. Build a complete new correct partition table. If your hard drive has errors

If your hard drive has errors, you may have real trouble mounting, checking or using data (the drive read errors get in the way). Gpart may not even find it. But if you know the start of the partition, you can easily copy the data to a temporary file stored on a different drive. Sectors with read errors will usually be set to zero by this process:

  • Copy the partition data to a file. You must know the start block of the partition;

dd if=/dev/hd?? of=/tmp/recover_hd?? bs=512 skip=XXXX count=YYY

XXX is the sector start and YYY the sector count (can be guessed).

  • Mount the file as a loop file system.

mount -r -t ext2 -o loop /tmp/recover_hd?? /mnt/recover

3.3. 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.