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GPT and the 2.2TB Barrier

Although most of the previous barriers in disk capacity have been hardware related, the 2.2TB barrier is more of a software than a hardware problem. Even more specifically, it is a disk formatting and OS problem, and it’s a BIOS problem if you consider boot drives versus data drives.

This problem stems from the way hard disks have been formatted since DOS 2.0 and the first PC hard drives appeared in 1983. Back then IBM and Microsoft came up with a scheme for partitioning drives called the MBR (Master Boot Record). The MBR is the first sector on a disk, and it is internally defined with the ability to control four primary partitions. Each partition is described by a 16-byte table entry, with 4-byte (32-bit) fields that define the LBA (Logical Block Address) for both where the partition starts and how big it is.

The largest number that can be written using 32 binary digits is 232, which is equal to 4,294,967,296. Because each sector is normally limited to 512 bytes, this means that the maximum amount of a drive that can be recognized is 2.2TB. Combine the MBR limitation with the fact that most PC BIOSs can only boot from MBR-formatted drives, and most older operating systems only support MBR-formatted drives for both boot drives and data drives, and you can see that the 2.2TB limitation can be a problem.

Several changes are necessary to break this barrier. The first is to develop a new partitioning scheme without the limitations the MBR imposes. This replacement is called GPT, which stands for GUID (globally unique identifier) Partition Table. Intel initially developed the GPT as part of its EFI (Extensible Firmware Interface) specification in 2000; since then Microsoft and other OS vendors have been incorporating it into operating systems. The GPT uses 64-bit LBA numbers, meaning disks of up to 9.4ZB (8ZiB) can be managed. That’s equal to 9.4 billion terabytes, a limit that won’t be reached any time soon.

Figure 7.19 illustrates the differences between MBR and GPT partitions.

Figure 7.19

Figure 7.19. GPT includes a backup for partition entries and the partition table header.

Although GPT breaks the 2.2TB barrier from a drive formatting perspective, other elements must be in place for GPT to be usable in a PC. To format or recognize a GPT-formatted disk, you need an OS that supports GPT. That alone allows you to use GPT-formatted disks as secondary (data) disks, but to boot from a GPT-formatted drive, you also need a motherboard with a UEFI BIOS or UEFI Boot option. Table 7.25 summarizes the requirements to break the 2.2TB barrier.

Table 7.25. Operating System GPT Boot/Data Disk Support

Operating System

GPT Boot Disk

GPT Data Disk

Windows XP (x86)

No

No1

Windows XP (x64)

No

Yes

Windows Vista SP1+ (x86)

No

Yes

Windows Vista SP1+ (x64)

Yes2

Yes

Windows 7/8 (x86)

No

Yes

Windows 7/8 (x64)

Yes2

Yes

Linux UBUNTU 8.04+/SUSE (x86, x64)

Yes

Yes

Note: x86 = 32-bit, x64 = 64-bit

1 Yes with third-party software such as the Paragon GPT Loader

2 Only on systems with a UEFI BIOS or an enabled UEFI Boot option

Drives 2.2TB or larger can also be supported externally via USB without having to resort to GPT partitioning. This is accomplished within the USB Bridge chipset firmware, which can be designed to present 2.2TB or larger drives using 4K sectors instead of the normal 512-byte sectors. USB enclosures or drive docks with this feature can use standard MBR formatting to allow the drive to be supported in Windows XP with no special software required.

In summary, to use a 2.2TB or larger drive as an internal secondary/data drive, you need to format it using GPT, and you need to run a GPT-aware OS (Vista SP1 or later). Booting from such a drive also requires a UEFI BIOS or an enabled UEFI Boot option. You can use GPT-formatted secondary/data drives with Windows XP by installing third-party GPT support software such as the Paragon GPT Loader (www.Paragon-Software.com).

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