HOWTO Recovering a Deleted Partition Table
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[edit] Introduction
This HOWTO will explain the process of recovering accidently deleted partition tables from linux partition, this has been tested with EXTFS so far, if you have tested this procedure with other file system types, please edit this paragraph.
[edit] Search for partition block.
Create a partition at least the same size as your first partition was. You can make it bigger than your old partition, what we do on the next step will identify the require block size. If you make the partition too small you'll just waste time. Run Fdisk on command prompt as under:
# Command (m for help): n Command action e extended p primary partition (1-4) # p # Partition number (1-4): 1 # First cylinder (1-45785, default 1): <HIT ENTER> Using default value 1 # Last cylinder or +size or +sizeM or +sizeK (1-42105, default 42105): 39002 # Command (m for help): w
[edit] Dumpe2fs Util
We will run dumpe2fs on the first partition and grep out the block count value.
- Example:
# dumpe2fs /dev/sda1 | grep "Block count:" Block count: 84253584
[edit] Verify Partition
If this value is not what you wanted, repeat Step 1 with a bigger partition size. If the block count changes, then you mis-calculated the size of the original partition. Repeat Step 1 until you get a correct block count.
- Remove the partition you just created
# Command (m for help): d
# Partition number (1-4): 1
[edit] Create Partition
Make a new partition with the exact size you got from the block count. Since you cannot enter block size in fdisk, you need to figure out how many cylinders to request. Here is the formula:
(number of needed cylinders) = (number of blocks) / (block size) (block size) = (unit size) / 1024 (unit size) = (number of cylinders) * (number of heads) * (number of sectors/cylinder) * (number of bytes/sector)
Consider the following example, where a hard drive has been partitioned into four primary partitions of 1, 2, 4, and 8 cylinders.
# disk /dev/sda 16 heads, 63 sectors, 23361 cylinders Units = cylinders of 1008 * 512 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 2 976+ 83 Linux
/dev/sda2 3 5 1512 83 Linux
/dev/sda3 6 10 2520 83 Linux
/dev/sda4 11 19 4536 83 Linux
fdisk provides the configuration information I need in the head of the output. The unit size is 516096 ( 16 heads * 63 sectors/cyl * 512 bytes/sector ). The block size is 504 ( 516096 / 1024 ). The number of needed cylinders for the second partition is therefore 3 ( 1512 blocks / 504 ). The partition table shows that this is indeed the case: the first cylinder is 3, the second 4, and the last is 5, for a total of three cylinders. The number of needed cylinders for the third partition is calculated similarly: 2520 blocks / 504 = 5, which corresponds to blocks 6,7,8,9,10 . Notice that this calculation does not work for the first partition because the block count is wrong ( 976 instead of 1008 ). The plus sign indicates that not all the blocks are included in the fdisk value. When you try the calculation ( 976 / 504 ) you get 1.937. Knowing that the number of cylinders must be an integer, you can simply round up.
[edit] Verify Partitions
Run e2fsck on it to verify that you can read the new partition. Repeat Steps 1-5 on remaining partitions. Remount your partitions. Amazingly, all of your data will be there.
[edit] Alternatives
[edit] Testdisk
Multi-platform tool to check and undelete partition, supports reiserfs, ntfs, fat32, ext2/3 and many others. Also includes
# emerge testdisk -av
[edit] PhotoRec
Recover pictures from digital camera memory Photo_Recovery_From_Defective_Memory_Card.
[edit] Credits
- Mike Vevea, jedi sys admin and MGH's finest, for giving me these tips.
[edit] Changelog
- 20080721 - Initial release. :Likewhoa
