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What is GPT (GUID Partition Table) Disk and Advantages of It?


Before briefly introducing some characteristics of GPT disk, you should known that partition information is stored directly in the master boot record (MBR), at the meantime, MBR also stores the information for loading operating system. But in the GPT hard disk, the location information of partition table is stored in the GPT header. But for compatibility reasons, the first sector of the hard disk is still used as a MBR, then the head of the GPT.

Like the traditional MBR, GPT also uses logical block address (LBA) instead of the early CHS address. The traditional MBR information stored in LBA 0 while GPT stored in the LBA 1, the next is the partition table itself. The 64 bit Windows operating system uses 16384 bytes (or 32 sectors) as the GPT partition table. The first partition on the hard disk starts at LBA 34.



The partition style of GPT disk is shown in the above figure. The Secondary GPT is a backup of the Primary GPT in case of the primary partition table being destroyed by accidents. At the begining of GPT partition table, for safety reasons, it stores a traditional MBR partition table to prevent the the disk management tool which doesn't support GPT from incorrectly recognizing GPT disk and destroying the data unexpectedly, it is called Protective MBR. This part also be used for the storage of startup code for the operating systems which support GPT boot disk.

Due to the Protective MBR, the operating system which cannot recognize GPT disk will identify it as an unknown type of partition and refuses to operate on the disk, unless the user specifically requested to delete the partition. This avoids the risk of accidentally deleting partition. In addition, the operating system which can recognize GPT partition table will check the protective MBR before it take any operations. If the partition type isn't 0xEE or the MBR partition table has more than one entry, it will refuse to operate on the disk.

For the disks which use MBR/GPT mixture partition table, in the same part of Protective MBR, it stores a few GPT partitions (usually the first four partitions of the entire GPT partition table). It enables the operating system that doesn't support for booting from GPT disk to boot from the MBR partitioning and can only operate on the partitions in the MBR partition table (e.g. Boot Camp uses this way to boot Windows).

The partition table header of GPT disk
Partition table header defines the disk space available and the composition of the partition table item size and quantity. For the computers which run 64 bit version of Windows operating system, you can create up to 128 partitions, namely the partition table retained 128 items, each of the items is 128 bytes (EFI standard partition table minimum to 16384 bytes, or 128 partitions item size). Partition table header records the partition ID of GPT disk and the partition table header size and location as well as the backup partition table header and the position and size of the partition table. It also stores the partition table CRC32. Firmware, boot program and the operating system can determine the partition table error according to the calibration value at startup. If there is any error found, you can use software to recover the whole hard disk partition table from the Secondary GPT. However, if the backup GPT parity error, the hard disk will not be available, so it isn't supported directly using 16 hex editor to modify the GPT hard disk partition table.

The advantages and benefits of GPT disk
Compare with MBR partitioning style, GPT disk has more advantages. It allows each disk to have up to 128 primary partitions, and the maximum volume size can grow up to 18 petabytes, allowing using primary and backup partition table for redundancy. Also, it supports each GPT partition to have a unique identification ID (GUID). However, the maximum volume size supported by MBR disk is only 2 TB (terabytes) and each disk can only has at most 4 primary partitions (or 3 primary partition + 1 extended partition, in this way unlimited logical partitions can be created on extended partition). In addition, unlike the MBR disk, the crucial platform operation data are stored in partitions but not in the unpartitioned place or hidden sector. In addition, GPT disk district have a surplus of primary and backup partition table to improve the integrity of regional data structure. Learn more differences between MBR and GPT disk.