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Disks: MBR, GUID, Basic, Dynamic

Managing disks in Windows isn’t that complicated, but there are some useful things you should know that can help you make smart decisions in allocating disk storage. Besides, you can never tell when this stuff might come up in a rousing game of Trivial Pursuit, MCSE Edition.

Partitions

When a computer boots up it doesn’t really know a whole lot about itself. It has to depend on other processes and information to get the party started. For instance, it might read the BIOS data to determine which hard disk is supposed to kick things off. But then what?

At this point the computer reads a small database that tells it how the required hard disk is configured. Using the information it learns here it can locate the operating system’s boot loader and start the process of getting the system up and running. Traditionally, this database was the disk’s partition table, a little 512-byte data structure located at the beginning of the drive.

Partition Table Structure

A classic partition table has four entries, each referring to one disk partition. A primary partition is a contiguous area of space that is logically treated as a volume, or disk of its own. The table limits each physical drive to four such volumes, but tricky geeks have sidestepped this apparent limitation by defining that no more than one of the partitions can be an extended partition—one that can be subdivided into multiple, non-bootable volumes.

This results in a disk with a maximum of four primary partitions, or three primary partitions and one extended partition.

Primary partitions can be made bootable, but cannot be subdivided into logical volumes. Extended partitions can be divided into many logical volumes, but cannot be bootable. This legacy design, introduced in the early 1980s, is called the MBR, or Master Boot Record. In most cases it’s limited to drives up to two terabytes in size. This is a limit imposed by the number of addressable sectors on the disk. Some manufacturers have introduced disks with larger sector sizes, allowing for MBR disks up to eight terabytes in size.

GPT, the GUID Partition Table

The GUID Partition Table was introduced to overcome some of the limitations of the traditional MBR. This table has space for up to 128 partition entries, and can be used with disks up to 18 terabytes on modern, 64-bit operating systems.

Basic Disks vs. Dynamic Disks

Windows defines two types of disks, basic and dynamic. A basic disk is limited to the partitions supported by the underlying table. It is simply the operating system’s view of what the disk says that it’s capable of doing. For example, if you’ve got an MBR partitioned disk, a basic disk allows you to create up to four partitions, one of which may be an extended partition.

Dynamic disks are much more flexible. Once a basic disk has been converted to a dynamic disk, you can sub-divide it any way you like, without regard to the drive’s partitioning scheme. You may even create spanned volumes, logical volumes that reside across multiple physical disks. Dynamic disks can also be used to create software RAID volumes.

Conversion

While basic disks can be converted to dynamic disks without data loss, the reverse is not true. Reverting to a basic disk requires that all logical volumes be removed from the dynamic disk first, thus resulting in the destruction of all information on the disk.

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