MBR is the standard partitioning scheme that’s been used on hard disks since the PC first came out. It supports 4 primary partitions per hard drive, and a maximum partition size of 2TB.
GPT disks are new, and are readable only by Windows Server 2003 SP1, Windows Vista (all versions), and Windows XP x64 Edition. The GPT disk itself can support a volume up to 2^64 blocks in length. (For 512-byte blocks, this is 9.44 ZB – zettabytes. 1 ZB is 1 billion terabytes). It can also support theoretically unlimited partitions.
Windows restricts these limits further to 256 TB for a single partition (NTFS limit), and 128 partitions.
Only Itanium systems running Windows Server 2003 and Windows Vista systems with an EFI BIOS can boot from a GPT disk. The other operating systems mentioned earlier can use GPT disks as data disks but not boot disks.
MBR Disk Layout
The following diagram from the Microsoft TechNet Library provides an example of a typical MBR disk layout
Figure 1: MBR Disk Layout
Perhaps one of the biggest pitfalls of MBR-based disks is their potential for corruption of the partition table, a region on the disk that maps sectors to logical block numbers. MBR disks only have 1 partition table to keep track of all the blocks in the partition. If the table becomes corrupt, the entire disk must be recovered from backup. Windows GPT-based disks have multiple, redundant partition tables so that if one is detected as being corrupt, it can self-heal itself from a redundant copy of the table.
For compatibility purposes, the Master Boot Record is kept at LBA 0 in GPT-based drives, and the GPT header begins at LBA 1. The partition type of a GPT disk is marked as 0xEE, which prevents MBR-based disk utilities from recognizing the partition type and potentially corrupting the data. It is possible to convert an MBR disk to a GPT-based disk and vice versa, but any data must first be backed up and all the partitions deleted.
GPT-based Disk Layout
The following diagram from the Microsoft TechNet Library provides an example of the disk layout for a GPT-based disk.
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