A hard drive can be configured for either MBR or GPT partitioning/booting. Not both.
GPT allows up to 2^64 (roughly 18,447,000,000,000,000,000 decimal) 512 byte sectors on a disk. GPT does not use Extended partitions, and does not permit hidden/unpartitioned areas on the disk. GPT stores its data in the first 34 sectors of the disk and maintains a backup (alternate) copy in the last 34 sectors. A CRC is included. The format includes version number and size information with the intent that future changes can be made transparently. Partitions include a Partition identifier with partition type and a 36 character unicode name in order to help resolve any ambiguities about partition identification and formatting. It includes a "protective" MBR style partition table that will show MBR oriented software a disk that is entirely used by a single type EE partition. This may allow a GPT formatted disk to be accessed as a secondary drive in an MBR BIOS computer at least to the extent of recognizing what it is.
Microsoft's 64 bit Windows XP recognizes two types of disks -- basic and dynamic. It appears that Basic GPT disks use the GPT to store volumes much as MBR disks do. Dynamic disks apparently define Microsoft specific partitioning within "container" partitions in the GPT.
GPT formatted hard drives may have more partitions for some OSes than equivalent MBR formatted drives would have. This is because information hidden in areas not allocated via the MBR partition table must be brought out into visible partitions on GPT disks.
Linux supports GPT in the kernel and the parted utility.
As with many new things, there are some issues involving things like corrupt GPT disks, how to handle disks made according to early versions of the specification where the protective MBR was optional, etc.
For information about the EFI standard, see http://msdn.microsoft.com/en-us/windows/hardware/default.aspx
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