How-to Fix Invalid MSDOS Partition Tables
Summary
This article describes how to fix some common problems that cause an
MSDOS partition table (also known as Master Boot Record - MBR) to be
invalid.
Note that a computer with an invalid partition table can often still
boot up. However, tools like GParted will not work until the
partition table is corrected.
|
CAUTION:
Editing partitions has the potential to cause LOSS of DATA. |
Contents
- Basic MSDOS Partition Table Rules
- Symptoms of an Invalid Partition Table
- Instructions (step-by-step)
MSDOS Partition Table Rules
|
TIP:
Primary and extended partions are identified by numbers 1 to 4
(e.g., /dev/sdb3) |
A valid MSDOS partition table must obey all of the following rules:
- At most four primary partitions, or three primary partitions and one extended partition are permitted.
- Primary and Extended partitions must not overlap.
- Primary partitions must not exist within an Extended partition.
-
An extended partition may contain multiple logical partitions.
It is permissible for a logical partition to end on the same sector as the end of the extended partition. - Logical partitions must not overlap other logical partitions.
-
At least 2 unallocated sectors are required in front of a
logical partition.
This space is required for the Extended Boot Record (EBR). -
All partitions must exist within the size of the disk device.
Sector numbering begins at zero, so the first sector is sector 0. -
Each partition must start within the first 2^32 (4,294,967,296)
sectors of the disk device, and be smaller than 2^32 sectors.
For disks with 512 byte sectors, 2^32 sectors is 2 Tebibytes (2 TiB).
Symptoms of an Invalid Partition Table
You might have an invalid partition table if you notice either of the following symptoms when using GParted:
- GParted shows the entire disk device as unallocated
- GParted fails when applying an action on a logical or an extended partition
Entire Disk Device Shown as Unallocated
There are a few situations that can cause this problem. To learn the cause, in GParted double-click on the "unallocated" disk device and check the information window for warning messages.
Common warning messages that cause this problem are as follows:
-
Can't have overlapping partitions
The root cause is that the partition table contains at least one occurrence where the end of one partition overlaps the start of another partition. See How-to Fix Overlapping Partitions.
-
Can't have a partition outside the disk!
The root cause is that the partition table contains at least one occurrence where the end of one partition is beyond the end of the disk device. See How-to Fix Partition Outside the Disk.
-
/dev/sda: unrecognized disk label
Two root causes for this message are as follows:
-
The device does not contain a partition table.
If the device does contain data then perhaps the entire device is formatted with a file system. It is also possible that the device is part of some other data structure, such as a RAID. In these situations there is no partition table to edit.
Otherwise, if this is a new disk device then the device is most likely empty. In this case you would proceed to create a partition table and at least one partition to prepare the disk device for use.
-
The type of partition table is not recognized by GParted.
If the device contains data, then perhaps the device is using a new type of partition table. GParted supports many commonly used partition tables, such as msdos or gpt, but not all types of partition tables are supported. In this situation you would need to determine what type of partition table is in use. Then you could seek out other partition management tools to edit the partition table.
-
The device does not contain a partition table.
Applying Action on Logical or Extended Partition Fails
To determine if the failure is due to an invalid partition table,
expand the GParted details in the "Applying pending operations"
window, and search for the source of the error.
If the text "Unable to satisfy all constraints on the
partition" is found, then there might be less than 2 unallocated
sectors in front of each logical partition.
For example, there should be at least 2 unallocated sectors between:
- the start of the extended partition and the start of a logical partition.
- the end of one logical partition and the start of the next logical partition.
The 2 unallocated sectors are required to store the Extended Boot
Record (EBR). The EBR contains information about the following
logical partition. The Linux kernel considers EBRs to be two sectors
long, in order to "leave room for LILO (LInux LOader)".
See How-to Fix Unable to Satisfy
All Constraints.
Instructions (step-by-step)
|
TIP: The sudo prefix is used to acquire root privilege. |
How-to Fix Overlapping Partitions
The following instructions describe how to manually correct the problem
of overlapping partitions.
Other methods also exist, such as
using testdisk
to scan the disk device to rebuild the partition table. The testdisk
application is included on GParted Live.
|
NOTE:
Be sure to choose the correct disk device path. |
- Ensure you have a good backup of your data.
-
Confirm the problem by running parted on your disk device
(e.g., /dev/sda).
For example:$ sudo parted /dev/sda unit s print Error: Can't have overlapping partitions.You should see the error message Error: Can't have overlapping partitions. -
Gather partition details for analysis using the fdisk command.
For example:$ sudo fdisk -l -u /dev/sda Disk /dev/sda: 60.0 GB, 60011642880 bytes 255 heads, 63 sectors/track, 7296 cylinders, total 117210240 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x000b2d37 Device Boot Start End Blocks Id System /dev/sda1 * 63 81922347 40961142+ 7 HPFS/NTFS/exFAT /dev/sda2 81915435 117210239 17647402+ 5 Extended /dev/sda5 81915498 98494514 8289508+ 83 Linux /dev/sda6 98494578 99008594 257008+ 82 Linux swap / Solaris /dev/sda7 99008658 115394894 8193118+ b W95 FAT32 -
Check the fdisk output for the following problems:
-
Do any of the primary or extended partitions overlap?
In other words does one partition (numbers 1 to 4) have a start value lower than another partitition (numbers 1 to 4) but also an end value higher than the start of the other partition?
-
Do any of the primary partitions exist within the extended
partition?
In other words does one partition (numbers 1 to 4) have start and end values within the boundaries of another partitition (number 1 to 4)?
If this is the situation then either the extended partition must be altered so that it does not encompass the primary partition, or the primary partition must be changed to be a logical partition within the extended partition. Fixing this situation might require backing up the partitions, repartitioning the disk, restoring the partition contents, and then repairing the ability to boot.
-
Do any of the logical partitions start or end outside the
extended partition?
In other words is the start of a logical partition (numbers 5+) less than the start of the extended partition?
Also, is the end of a logical partition (numbers 5+) greater than the end of the extended partition?
-
Do any of the logical partitions overlap?
In other words does one logical partition (numbers 5+) have a start value lower than another logical partition (numbers 5+) but also have an end value higher than the start of the other partition?
81,922,347 --- end of sda1
81,915,435 --- start of sda2
Device Boot Start End Blocks Id System /dev/sda1 * 63 81922347 40961142+ 7 HPFS/NTFS/exFAT /dev/sda2 81915435 117210239 17647402+ 5 Extended /dev/sda5 81915498 98494514 8289508+ 83 Linux /dev/sda6 98494578 99008594 257008+ 82 Linux swap / Solaris /dev/sda7 99008658 115394894 8193118+ b W95 FAT32 -
Do any of the primary or extended partitions overlap?
-
Determine a new end value (and partition size) to avoid the
overlap.
NOTE:
- In cases where we do not know if the starting or ending sector is the problem, we assume that the starting sector of each partition is correct, and that the ending sector might be in error.
- No sectors are required between primary and extended partitions.
- At least 2 sectors of unallocated space are required in front of logical partitions.
In this example, we assume that the starting sector of extended partition sda2 is correct. Hence we need to change the end sector of primary partition sda1.
The new end sector of sda1 should be the start of sda2 minus one sector.
* For logical partitions we would need to subtract at least two (2) sectors.new sda1 end = (sda2 start) - 1 = 81915435 - 1 = 81915434New size of sda1 will be the new end of sda1 minus the start of sda1 plus one sector.new sda1 size = (new sda1 end) - (sda1 start) + 1 = 81915434 - 63 + 1 = 81915372 -
Make a copy of the partition table in an editable file using the
sfdisk command.
For example:
$ sudo sfdisk -d /dev/sda > sda-backup.txt -
Use your favourite editor to edit the file to change the old
partition size to the new partition size.
If you are using GParted Live, you can edit the file using the Leafpad editor.
For example:
$ sudo leafpad sda-backup.txt # partition table of /dev/sda unit: sectors /dev/sda1 : start= 63, size= 81922285, Id= 7, bootable /dev/sda2 : start= 81915435, size= 35294805, Id= 5 /dev/sda3 : start= 0, size= 0, Id= 0 /dev/sda4 : start= 0, size= 0, Id= 0 /dev/sda5 : start= 81915498, size= 16579017, Id=83 /dev/sda6 : start= 98494578, size= 514017, Id=82 /dev/sda7 : start= 99008658, size= 16386237, Id= bChange the old size of the primary partition sda1 (81922285) to the calculated new size (81915372).# partition table of /dev/sda unit: sectors /dev/sda1 : start= 63, size= 81915372, Id= 7, bootable /dev/sda2 : start= 81915435, size= 35294805, Id= 5 /dev/sda3 : start= 0, size= 0, Id= 0 /dev/sda4 : start= 0, size= 0, Id= 0 /dev/sda5 : start= 81915498, size= 16579017, Id=83 /dev/sda6 : start= 98494578, size= 514017, Id=82 /dev/sda7 : start= 99008658, size= 16386237, Id= bSave the file and exit the editor. -
Write the corrected partition details to the partition table using
the sfdisk command.
For example:
$ sudo sfdisk /dev/sda < sda-backup.txt Checking that no-one is using this disk right now ... OK Disk /dev/sda: 7296 cylinders, 255 heads, 63 sectors/track Old situation: Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0 Device Boot Start End #cyls #blocks Id System /dev/sda1 * 0+ 5099- 5100- 40961142+ 7 HPFS/NTFS/exFAT /dev/sda2 5099 7295 2197 17647402+ 5 Extended /dev/sda3 0 - 0 0 0 Empty /dev/sda4 0 - 0 0 0 Empty /dev/sda5 5099+ 6130 1032- 8289508+ 83 Linux /dev/sda6 6131+ 6162 32- 257008+ 82 Linux swap / Solaris /dev/sda7 6163+ 7182 1020- 8193118+ b W95 FAT32 New situation: Units = sectors of 512 bytes, counting from 0 Device Boot Start End #sectors Id System /dev/sda1 * 63 81915434 81915372 7 HPFS/NTFS/exFAT /dev/sda2 81915435 117210239 35294805 5 Extended /dev/sda3 0 - 0 0 Empty /dev/sda4 0 - 0 0 Empty /dev/sda5 81915498 98494514 16579017 83 Linux /dev/sda6 98494578 99008594 514017 82 Linux swap / Solaris /dev/sda7 99008658 115394894 16386237 b W95 FAT32 Successfully wrote the new partition table Re-reading the partition table ... If you created or changed a DOS partition, /dev/foo7, say, then use dd(1) to zero the first 512 bytes: dd if=/dev/zero of=/dev/foo7 bs=512 count=1 (See fdisk(8).) -
Confirm the problem is now resolved by running parted on your disk
device.
For example:$ sudo parted /dev/sda unit s print Model: ATA ST3060022ACE (scsi) Disk /dev/sda: 117210240s Sector size (logical/physical): 512B/512B Partition Table: msdos Number Start End Size Type File system Flags 1 63s 81915434s 81915372s primary ntfs boot 2 81915435s 117210239s 35294805s extended 5 81915498s 98494514s 16579017s logical ext2 6 98494578s 99008594s 514017s logical linux-swap(v1) 7 99008658s 115394894s 16386237s logical fat32You should now be able to use GParted with this device. :-)
How-to Fix Partition Outside the Disk
The following instructions describe how to manually correct the problem
of a partition extending beyond the end of the disk.
Other methods also exist, such as
using testdisk
to scan the disk device to rebuild the partition table. The testdisk
application is included on GParted Live.
If the problem occurs with the extended partition, then you might
consider using
the fixparts
application. The fixparts application is also included
on GParted Live.
|
NOTE:
Be sure to choose the correct disk device path. |
- Ensure you have a good backup of your data.
-
Confirm the problem by running parted on your disk device
(e.g., /dev/sdb).
For example:$ sudo parted /dev/sdb unit s print Error: Can't have a partition outside the disk!You should see the error message Error: Can't have a partition outside the disk! -
Gather partition details for analysis using the fdisk command.
For example:$ sudo fdisk -l -u /dev/sdb Disk /dev/sdb: 250.1 GB, 250059350016 bytes 255 heads, 63 sectors/track, 30401 cylinders, total 488397168 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00068df3 Device Boot Start End Blocks Id System /dev/sdb1 * 63 12578894 6289416 7 HPFS/NTFS/exFAT /dev/sdb2 12578895 180345689 83883397+ 7 HPFS/NTFS/exFAT /dev/sdb3 180345690 390058199 104856255 7 HPFS/NTFS/exFAT /dev/sdb4 390058200 488408189 49174995 5 Extended /dev/sdb5 390058263 459089504 34515621 83 Linux /dev/sdb6 459089568 488392064 14651248+ 82 Linux swap / Solaris -
Check the fdisk output for the cause of the problem.
Does any partition have an end value larger than the disk size?
To be precise, since the first sector begins at sector zero (0), check to see if the end of any partition is greater than the disk size minus one sector.
In this example the end of extended partition sdb4 is beyond the size of the disk.
488,408,189 --- end of sdb4
488,397,168 --- size of sdb device
Disk /dev/sdb: 250.1 GB, 250059350016 bytes 255 heads, 63 sectors/track, 30401 cylinders, total 488397168 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00068df3 Device Boot Start End Blocks Id System /dev/sdb1 * 63 12578894 6289416 7 HPFS/NTFS/exFAT /dev/sdb2 12578895 180345689 83883397+ 7 HPFS/NTFS/exFAT /dev/sdb3 180345690 390058199 104856255 7 HPFS/NTFS/exFAT /dev/sdb4 390058200 488408189 49174995 5 Extended /dev/sdb5 390058263 459089504 34515621 83 Linux /dev/sdb6 459089568 488392064 14651248+ 82 Linux swap / Solaris -
Determine a new end value (and partition size) to prevent the
partition outside the disk problem.
The new end sector of the partition outside the disk should be the size of the disk minus one sector.
In our example:new sdb4 end = (sdb disk size) - 1 = 488397168 - 1 = 488397167New size of sda1 will be the new end of sda1 minus the start of sda1 plus one sector.new sdb4 size = (new sdb4 end) - (sdb4 start) + 1 = 488397167 - 390058200 + 1 = 98338968 -
Make a copy of the partition table in an editable file using the
sfdisk command.
For example:
$ sudo sfdisk -d /dev/sdb > sdb-backup.txt -
Use your favourite editor to edit the file to change the old
partition size to the new partition size.
If you are using GParted Live, you can edit the file using the Leafpad editor.
For example:
$ sudo leafpad sdb-backup.txt # partition table of /dev/sdb unit: sectors /dev/sdb1 : start= 63, size= 12578832, Id= 7, bootable /dev/sdb2 : start= 12578895, size=167766795, Id= 7 /dev/sdb3 : start=180345690, size=209712510, Id= 7 /dev/sdb4 : start=390058200, size= 98349990, Id= 5 /dev/sdb5 : start=390058263, size= 69031242, Id=83 /dev/sdb6 : start=459089568, size= 29302497, Id=82Change the old size of the partition sdb4 (98349990) to the calculated new size (98338968).# partition table of /dev/sdb unit: sectors /dev/sdb1 : start= 63, size= 12578832, Id= 7, bootable /dev/sdb2 : start= 12578895, size=167766795, Id= 7 /dev/sdb3 : start=180345690, size=209712510, Id= 7 /dev/sdb4 : start=390058200, size= 98338968, Id= 5 /dev/sdb5 : start=390058263, size= 69031242, Id=83 /dev/sdb6 : start=459089568, size= 29302497, Id=82Save the file and exit the editor. -
Write the corrected partition details to the partition table using
the sfdisk command.
For example:
$ sudo sfdisk /dev/sdb < sdb-backup.txt Checking that no-one is using this disk right now ... OK Disk /dev/sdb: 30401 cylinders, 255 heads, 63 sectors/track Old situation: Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0 Device Boot Start End #cyls #blocks Id System /dev/sdb1 * 0+ 782 783- 6289416 7 HPFS/NTFS/exFAT /dev/sdb2 783 11225 10443 83883397+ 7 HPFS/NTFS/exFAT /dev/sdb3 11226 24279 13054 104856255 7 HPFS/NTFS/exFAT /dev/sdb4 24280 30402- 6123- 49174995 5 Extended /dev/sdb5 24280+ 28576 4297- 34515621 83 Linux /dev/sdb6 28577+ 30400 1824- 14651248+ 82 Linux swap / Solaris New situation: Units = sectors of 512 bytes, counting from 0 Device Boot Start End #sectors Id System /dev/sdb1 * 63 12578894 12578832 7 HPFS/NTFS/exFAT /dev/sdb2 12578895 180345689 167766795 7 HPFS/NTFS/exFAT /dev/sdb3 180345690 390058199 209712510 7 HPFS/NTFS/exFAT /dev/sdb4 390058200 488397167 98338968 5 Extended /dev/sdb5 390058263 459089504 69031242 83 Linux /dev/sdb6 459089568 488392064 29302497 82 Linux swap / Solaris Warning: partition 4 does not end at a cylinder boundary Warning: partition [6] does not end at a cylinder boundary Successfully wrote the new partition table Re-reading the partition table ... If you created or changed a DOS partition, /dev/foo7, say, then use dd(1) to zero the first 512 bytes: dd if=/dev/zero of=/dev/foo7 bs=512 count=1 (See fdisk(8).) -
Confirm the problem is now resolved by running parted on your disk
device.
For example:$ sudo parted /dev/sdb unit s print Model: ATA ST3250022ACE (scsi) Disk /dev/sdb: 488397168s Sector size (logical/physical): 512B/512B Partition Table: msdos Number Start End Size Type File system Flags 1 63s 12578894s 12578832s primary ntfs boot 2 12578895s 180345689s 167766795s primary ntfs 3 180345690s 390058199s 209712510s primary ntfs 4 390058200s 488397167s 98338968s extended 5 390058263s 459089504s 69031242s logical ext2 6 459089568s 488392064s 29302497s logical linux-swap(v1)You should now be able to use GParted with this device. :-)
How-to Fix Unable to Satisfy All Constraints
We plan to outline how to approach this problem. In the meantime you can seek help in the GParted forum.
