For decades, MBR partitioning was sufficient to support the amount of hard drive addressing spaces known as Logical Block Addresses (LBAs) that existed on hard drives. MBR partitioning supported 32-bit addressing and, along with 48-bit LBA extensions, was able to support drives up to a limit of 2.19TB using standard 512- byte sectors (or 512-byte sector emulation). However, now that hard drive sizes are beginning to exceed the 2.19TB limit, a newer approach is needed. GPT partitioning, which is part of the overall Unified Extensible Firmware Interface (UEFI) specification, allows for much larger hard drives than with MBR and will also allow such a drive to be bootable in a system with UEFI BIOS and UEFI-aware OS.
GPT vs MBR
There are a number of differences between MBR and GPT. Some of the key differences include the following:
•MBR only supports 4 primary partition table entries while GPT supports 128 primary partitions with current Microsoft Windows™ operating systems.
•MBR uses 32-bit values to indicate the starting offset and length of a partition. Assuming 512 byte sectors or 512 byte sector emulation, this means an MBR disk supports 2^32 bit LBAs multiplied by 512 bytes/sectors = 2.19 x10^12 bytes = 2.19TB maximum disk size.
•Since GPT uses 64-bit values to indicate the starting offset and length of a partition, a GPT disk can theoretically support up to 2^64 LBAs. Assuming 512 byte sectors or 512 byte sector emulation, this means the maximum capacity of a GPT disk = 9.4 x 10^21 bytes = 9.4 zettabytes (ZB). Microsoft Windows™ file systems design currently limits this to 256TB per drive
•GPT provides redundancy by writing the GPT header and partition table at the beginning of the disk and also at the end of the disk. If the original GPT header and partition table gets deleted or corrupted, the backup header and partition table can be used to help restore the drive. The contents of the GPT header and partition table are also verified via a CRC32 checksum for the header and one for the partition table.
Figure 1 on the following page shows a diagram of the GPT Partition Table and drive layout.