Where Is the Primary GPT Header Located?

This article explains the exact physical and logical location of the primary GUID Partition Table (GPT) header on a storage drive managed by UEFI. You will learn about its position within the Logical Block Addressing (LBA) scheme, why it resides immediately after the protective Master Boot Record, and the structural role it plays in managing storage partitions on modern systems.

The Exact Location: Logical Block Address 1 (LBA 1)

On any storage device initialized with a standard GUID Partition Table under UEFI specifications, the primary GPT header is located at LBA 1 (Logical Block Address 1).

In standard modern storage systems using 512-byte sector emulation (512e) or legacy 512-byte native sectors, LBA 1 corresponds to byte offset 512 to 1023 on the physical disk. On native 4K (4Kn) advanced format drives, LBA 1 corresponds to byte offset 4096 to 8191. Regardless of the physical sector size, the primary GPT header always occupies the second logical block on the disk.

The Surrounding Disk Layout

To understand why the primary GPT header is located at LBA 1, it helps to examine the contiguous layout of the disk:

  • LBA 0: Protective MBR: The first block (LBA 0) is reserved for a Protective Master Boot Record (MBR). This legacy structure contains a single partition entry of type 0xEE spanning the entire disk. Its purpose is to prevent legacy disk management utilities that do not recognize GPT from identifying the disk as unpartitioned and accidentally overwriting data.
  • LBA 1: Primary GPT Header: Immediately following the Protective MBR is the primary GPT header itself. It defines the disk's layout, identifying partition boundaries and integrity checks.
  • LBA 2 to 33: Partition Entry Array: Directly after the primary header sits the table of partition entries. By default, UEFI specifications allocate a minimum of 16,384 bytes for partition entries, accommodating at least 128 partition records (each 128 bytes in size), spanning from LBA 2 through LBA 33.

Contents of the Primary GPT Header

The primary header at LBA 1 serves as the central directory for the storage device. It contains crucial metadata that the UEFI firmware reads during initialization:

  • Signature: An 8-byte ASCII string reading "EFI PART" (bytes 45 46 49 20 50 41 52 54) to identify the GPT structure.
  • Revision: The specific UEFI specification version supported by the header.
  • Header Size and Checksum: The size of the header (typically 92 bytes) and a CRC32 checksum of the header itself to detect corruption.
  • Current LBA: The block location of the header itself (recorded as 1).
  • Backup LBA: The logical address of the secondary (backup) GPT header, located at the very last accessible block of the storage device.
  • First Usable LBA and Last Usable LBA: Defines the block range available for actual data partitions.
  • Disk GUID: A unique identifier assigned to the entire disk.
  • Partition Entry Starting LBA: Points to LBA 2, the start of the partition array, along with the total number of partition entries and the size of each entry.

Redundancy and Recovery

Because the primary GPT header at LBA 1 is critical for reading the partition structure, the UEFI standard mandates a redundant backup. The secondary GPT header is placed at the final sector of the disk (LBA -1), preceded by a backup copy of the partition entry array. If the primary header at LBA 1 is corrupted or fails its CRC32 checksum, UEFI firmware automatically references the secondary header at the end of the drive to recover the partition scheme.