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A GUID Partition Table (GPT) is a disk partitioning scheme defined by the UEFI specification. It records where partitions begin and end, along with identifiers and other metadata. GPT is not a filesystem: a partition can use its own filesystem while GPT describes the partition’s place on the disk.
What GPT records on a disk
GPT organizes a disk using logical block addresses (LBAs), which identify locations in the disk’s addressable blocks. Its header points to an array of partition entries. Each in-use entry records a partition’s starting and ending LBAs, partition-type GUID, unique partition GUID, attributes, and a human-readable name. The partition type identifies the intended kind of partition; the unique GUID distinguishes that particular partition.
The UEFI Specification 2.10 describes the format’s fields and layout in section 5, GUID Partition Table (GPT) Disk Layout. The same layout is corroborated by the official UEFI Specification 2.11 PDF.
Where the GPT structures sit
In the UEFI-defined layout, the first logical block is LBA 0, the second is LBA 1, and the backup header is at the disk’s last LBA. The structures are arranged as follows:
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- LBA 0 — Protective MBR: A protective Master Boot Record helps older tools recognize that the disk contains a partitioning scheme they may not understand. It is not the primary GPT.
- LBA 1 — Primary GPT header: This header starts with the
EFI PARTsignature. It records, among other fields, the header revision and size, its own LBA, the alternate header’s LBA, usable LBA bounds, a disk GUID, and the location and dimensions of the partition-entry array. - Partition-entry array: The array holds the entries that describe partitions. Its location, entry count, entry size, and CRC32 value are recorded in the header.
- Last LBA — Backup GPT header: GPT stores a backup header at the disk’s final logical block. The primary and backup headers refer to each other.
The specification also defines backup partition-entry information. These structures provide another copy of partition metadata, not another copy of the files stored in the partitions.
How GPT checks metadata integrity
GPT uses CRC32 checks for the header and partition-entry array. When calculating a header’s CRC32, the checksum field itself is treated as zero. The array has a separate CRC32 value recorded in the header. If partition entries change, the array checksum and then the header checksum must be updated.
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A CRC mismatch can reveal certain metadata corruption; it does not guarantee recovery or show that partition contents are intact. The backup GPT structures may help recover partition metadata in some cases, but they are not a substitute for backing up files.
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| Feature | GPT | Legacy MBR |
|---|---|---|
| Address fields | 64-bit LBAs | 32-bit LBAs |
| Partition records | Supports many entries; the configured partition-entry array determines the number | Four partition records in the legacy MBR layout |
| Partition-table copies | Primary and backup GPT structures | No matching backup table in the described legacy layout |
| Identifiers and integrity fields | GUID identifiers and CRC32 fields | The cited UEFI comparison does not list equivalent GPT-style fields |
The comparison reflects the UEFI specification’s description of the formats. The number of partitions a system can actually use depends on the GPT array configuration and the software that reads it; support and limits should not be assumed identical across operating systems and tools.
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What GPT does not mean
- It is not a filesystem. GPT describes partition boundaries and metadata; a filesystem organizes files within a partition.
- It is not a backup of your files. Redundant partition structures concern partition metadata, not the contents of the disk.
- It does not, by itself, determine what every tool supports. The format permits many partition entries, but actual limits depend on the configured array and software support.
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