MBR Partition Table
What Is the MBR Partition Table?
The MBR Partition Table is a 64-byte structure stored inside the Master Boot Record (sector LBA 0) of an MBR-formatted disk. It defines how storage space is divided and tells the system where each partition begins and ends. Unlike GPT, which stores multiple redundant partition tables, the MBR Partition Table has only one copy, making it highly vulnerable to corruption.
The table contains four 16-byte partition entries, meaning an MBR disk supports up to four primary partitions (or three primaries + one extended partition containing logical volumes).
When the MBR Partition Table becomes damaged, the system cannot map partitions correctly, causing them to disappear, appear as RAW, or show incorrect size information.
Table of Contents
MBR Partition Table: Quick Facts
Feature | MBR Partition Table |
Full name | Master Boot Record Partition Table |
Location | LBA 0, the first logical sector of the disk |
Partition table size | 64 bytes |
Number of partition entries | 4 |
Size of each entry | 16 bytes |
Maximum primary partitions | 4 |
Additional partitions | Supported through an extended partition and logical drives |
Addressing fields | 32-bit Starting LBA and sector-count fields |
Common capacity limit | Approximately 2 TiB with 512-byte logical sectors |
Built-in backup partition table | No |
Built-in checksum | No |
Common boot environment | Legacy BIOS |
Where Is the MBR Partition Table Located?
The MBR partition table is stored at LBA 0, which is the first logical block of an MBR disk.
On a conventional disk with 512-byte logical sectors, the first sector is divided into the following regions:
Byte Offset | Size | Content |
0–445 | 446 bytes | Bootstrap code and related disk information |
446–461 | 16 bytes | Partition entry 1 |
462–477 | 16 bytes | Partition entry 2 |
478–493 | 16 bytes | Partition entry 3 |
494–509 | 16 bytes | Partition entry 4 |
510–511 | 2 bytes | MBR signature |
The four partition entries occupy exactly 64 bytes:
4 partition entries × 16 bytes = 64 bytes
Because the partition table occupies only a small part of the first sector, damage to LBA 0 can affect the disk layout without directly erasing all files stored in later sectors.
However, whether data remains recoverable depends on what caused the damage and whether additional data has been written to the disk.
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MBR Partition Table Layout
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MBR partition table layout showing boot code, four partition entries, and the 55AA signature
How Does the MBR Partition Table Work?
When a computer or operating system detects an MBR disk, it reads the partition entries stored in LBA 0.
Each valid entry describes a range of sectors assigned to a partition.
For example, an entry may indicate:
- The partition begins at LBA 2,048
- The partition contains 500,000,000 sectors
- The partition uses type code
0x07 - The partition is not marked as active
The operating system uses the starting LBA and partition size to calculate the partition boundaries.
The ending location can be calculated as:
Ending LBA = Starting LBA + Number of Sectors − 1
For example:
Starting LBA = 2,048
Number of Sectors = 1,000,000
Therefore:
Ending LBA = 2,048 + 1,000,000 − 1
Ending LBA = 1,002,047
The operating system then examines the data inside that sector range to identify and mount the file system.
The partition table and the file system are separate structures.
The partition table tells the operating system where a partition is located, while the file system organizes files and folders inside the partition.
This distinction is important during troubleshooting.
A partition may still have a valid MBR entry but contain a damaged file system. Conversely, a partition entry may be missing while much of the NTFS, FAT32, exFAT, or other file-system data remains present.
Why Does MBR Have a 2 TB Limit?
The commonly cited MBR disk limit is approximately 2 TiB when 512-byte logical sectors are used.
The limitation comes from the 32-bit sector-count fields in each MBR partition entry.
A 32-bit unsigned value can represent up to:
2³² sectors
With 512 bytes per logical sector:
2³² × 512 bytes = 2,199,023,255,552 bytes
This equals approximately:
- 2.2 TB in decimal units
- 2 TiB in binary units
For simplicity, operating-system documentation and storage product interfaces often describe this as a 2 TB MBR limit. Microsoft recommends GPT for disks larger than 2 TB.
The exact practical limit can vary with logical-sector size and software compatibility. However, GPT is generally the preferred partitioning scheme for modern high-capacity disks.
How to Repair an MBR Partition Table (Step-by-Step)
1. Avoid Formatting or Initializing the Disk
If Windows states “You need to format the disk” or prompts to initialize it as GPT/MBR, cancel immediately.
Formatting replaces partition entries, risking permanent metadata loss.
2. Inspect the Disk in Windows Disk Management
If the disk appears as RAW or Unallocated, the partition table may be unreadable.
But the data inside the partition region often remains untouched.
3. Use Windows Recovery Environment (for boot drives)
If the system cannot boot:
- Boot from Windows installation media
- Open Command Prompt
- Run:
bootrec /fixmbr
bootrec /fixboot
bootrec /scanos
bootrec /rebuildbcd
These commands repair the bootloader but do not rebuild the partition table.
They help only if the boot entry, not the table itself, is corrupted.
4. Rebuild the MBR Partition Table
Partition recovery tools like DiskDrill or Minitool can detect file systems and rebuild the partition table automatically. Make sure the tool doesn’t format the partitions in case of data loss.
5. Recover Data Before Making Structural Modifications
If the partition table is severely corrupted, the safest option is to extract data before rebuilding the table.
Tools like Magic Data Recovery read file system signatures directly, bypassing damaged MBR structures. Even if the disk appears RAW or unrecognized, the software can scan sectors, locate NTFS/MFT metadata, and recover intact files.
6. Avoid CHKDSK on Partition-Table-Damaged Drives
CHKDSK can modify or delete orphaned records and should not be used until the file system is confirmed consistent.
MBR Partition Table vs. GPT
GPT, or GUID Partition Table, is the modern alternative to MBR.
GPT uses 64-bit LBA fields, supports more partitions, stores primary and backup partition information, and uses CRC32 values to help detect corruption.
Feature | MBR | GPT |
Full name | Master Boot Record | GUID Partition Table |
Partition entries | 4 entries in the main table | Many entries; Windows commonly creates space for 128 |
Maximum primary partitions | 4 | More than 4 |
Additional partitions | Uses extended partitions and logical drives | No extended partition required |
Addressing | 32-bit LBA fields | 64-bit LBA fields |
Typical Windows disk-size limit | About 2 TB | Supports disks larger than 2 TB |
Primary partition-table location | LBA 0 | GPT header normally begins at LBA 1 |
Backup partition information | No standard backup table | Primary and backup GPT structures |
Integrity checking | No built-in partition-table checksum | CRC32 integrity checks |
Common Windows system boot environment | Legacy BIOS | UEFI |
Modern Windows system disks | Supported in compatible BIOS configurations | Generally preferred |
Large-capacity disks | Limited | Recommended |
GPT also includes a protective MBR at LBA 0. The protective entry helps prevent older MBR-only tools from incorrectly treating a GPT disk as empty.
Conclusion
The MBR Partition Table is essential for interpreting disk layout, and damage to this 64-byte structure can cause missing partitions, RAW disks, or boot failures.
Fortunately, the data inside the partitions is often still intact.
By avoiding formatting, using proper reconstruction methods, and recovering data before applying structural changes, users can safely restore access.
When the drive becomes unrecognized or RAW, Magic Data Recovery provides a reliable, read-only method to extract files before repairing the disk.
Supports Windows 7/8/10/11 and Windows Server
FAQs for MBR Partition Table
What is an MBR partition table?
Where is the MBR partition table located?
How many partitions can an MBR disk have?
Why is MBR limited to 2 TB?
Does BOOTREC /FIXMBR rebuild the MBR partition table?
Is a RAW partition the same as an Unallocated disk?
Can a damaged MBR partition table be repaired without losing data?
