GPT par­ti­tions: What is a GUID partition table?

Par­ti­tions are a tried-and-tested way of getting optimal per­form­ance from internal and external storage media. The division of storage space doesn’t just provide more structure, it also gives you the pos­sib­il­ity of sep­ar­at­ing system and ap­plic­a­tion files from one another in the best possible way. In this way, for example, all important data in the operating system can be stored in the first sectors of a hard drive, which usually offer the highest reading and writing speeds.

The technical found­a­tion for this par­ti­tion­ing is so-called par­ti­tion­ing schemes and tables that hold all relevant in­form­a­tion about the division of the data storage. One of the newest solutions for this is the standard GUID partition table, the found­a­tion for which we will deal with in more detail here.

GUID partition table (GPT) is a standard for format­ting partition tables for storage media, for hard drives. GPT is a component of UEFI (Unified Extensible Firmware Interface), an interface spe­cific­a­tion that governs exchange between firmware and operating systems during the boot process and that was developed and published in 2000 to replace BIOS. However, GPT par­ti­tions can also be used – with re­stric­tions – in­de­pend­ently of UEFI. In this case it is necessary that the operating system used and the hard drive to be par­ti­tioned support the standard. Owing to various im­prove­ments related to it, today GPT has largely replaced the tra­di­tion­al partition style, MBR (master boot record).

Using GPT as a partition style for hard drives is re­com­men­ded for a wide variety of reasons. At the top of the list is the fact that a GUID partition table relies on entries with 64 bits for its ad­dress­ing. This means that the maximum size of a partition is exactly 18 exabytes, which cor­res­ponds to 18 billion gigabytes. For today’s gen­er­a­tion of storage media, which even in the private sector already offer several terabytes of storage space, this is an in­valu­able attribute. The older master boot record scheme only permits a partition size of two terabytes at most, which for many modern hard drives is simply too little. The other prop­er­ties and ad­vant­ages of GPT par­ti­tion­ing can be sum­mar­ised as follows:

• No limit for primary par­ti­tions: In theory, GUID partition table permits an unlimited number of primary par­ti­tions for struc­tur­ing storage space. In practice, operating systems set a limit. In Windows, for example, the value is 128, which is more than enough.
• Safe­guard­ing through CRC32 checksums: Checksums ensure the integrity necessary for the GPT header. Among other things, this technique detects if faulty sectors are damaging the header area.
• Clear iden­ti­fic­a­tion of par­ti­tions and data carriers: Thanks to the GUID tech­no­logy already mentioned, data carriers and par­ti­tions receive clear iden­ti­fic­a­tion numbers.
• Back-up header: The header line of GUID partition tables is not only secured with the checksum mentioned, but also with a congruent back-up. This increases the security of the partition metadata and minimises the risk of data loss in case of a hardware fault.
• Downward com­pat­ib­il­ity: A so-called pro­tect­ive master boot record in Sector 0, the first data block of a GPT hard drive, ensures that almost all operating systems, services and tools designed with MBR par­ti­tion­ing also work with GPT.

The header of the re­spect­ive GUID partition table structure describes the useful blocks in the data carrier and the size of the in­di­vidu­al partition entries, among other things. It is therefore in­dis­pens­able for the GPT par­ti­tions to work. As already becomes clear in the graphic rep­res­ent­a­tion of the scheme, the GPT header is always saved in the second sector of the storage medium (LBA 1) – directly after the pro­tect­ive MBR. The security copy of the header can also be found on the data carrier, though the final sector is stip­u­lated for this back-up (LBA -1). The exact positions of the two versions, which are both protected by a checksum, are also stored in the header.

Since 2005, GUID partition table have in­creas­ingly been gaining ground as the partition style for HDD and SSD hard drives. The reason behind this is prin­cip­ally the in­creas­ing use of UEFI as an al­tern­at­ive to BIOS: For this reason, modern hardware and operating systems more and more fre­quently use the new interface standard, and therefore GUID par­ti­tion­ing too. Apart from current editions of Windows like Windows 10, 8 or 7, various newer macOS and Linux versions also rank among the operating systems supported by GPT.

GUID partition tables can therefore be used con­veni­ently in almost all modern computer systems that work with in­teg­rated data carriers and whose storage capacity is in the gigabyte and terabyte range. The partition style is just as sought-after in external hard drives, of course – es­pe­cially since here many solutions now provide more than two terabytes of storage and therefore exclude MBR as partition tech­no­logy. Another typical area of ap­plic­a­tion for GPT: Bootable USB sticks. For this it always depends which operating system and firmware they are designed for, and whether UEFI (meaning GPT) or BIOS is to be used.

Of course, GPT par­ti­tion­ing is also a solution for USB sticks if the stick is not to be used as a boot medium, but as a simple storage medium. In par­tic­u­lar, because of the increased pro­tec­tion against data loss if there is a hardware fault, for which the risk is po­ten­tially higher with portable storage media (so also with external hard drives).