GPT (Globally Unique Identifier Partition Table) disks can be a better choice than MBR (Master Boot Record) in some situations. For one thing, while you can only have 4 partitions on an MBR disk, you can have up to 128 partitions on a GPT disk, and GPT disks support very large partitions up to 18 exabytes in size.
More importantly for server systems, GPT disks are designed to accurately map to the physical geometry of today's hard drives. This means for example that when Windows creates a partition on a GPT disk, it partitions the disk along cylinder boundaries, and this results in more reliable partitioning. In addition, GPT disks have redundant partition tables and CRC fields for even greater reliability.
Note that GPT disks can't be used as the system volume in BIOS-based systems—your system must support EFI to be able to boot from a GPT disk. But BIOS-based systems can use GPT disks as additional data drives.
To convert an existing MBR data drive to GPT, use the convert gpt command. To convert back, use the convert mbr command. The drive must have no partitions for this to work, and if you have an MBR disk that is a dynamic (not basic) disk you must first convert it to basic before converting it to GPT.
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