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The 64-bit Itanium versions of Windows use a different disk layout from the conventional 32-bit disk. The new structure, called GUID Partition Table (GPT) is more reliable and can handle larger disk partitions, but it is not backward-compatible with many programs and some Microsoft services.
One of the most important limitations is that Microsoft Cluster Server using Windows 2003 Server does not support GPT disks. This is particularly important because the Itanium processor is aimed at the server market.
If you try to install server clusters on GPT disks, the Configuration Wizard will not detect them as available for use. You cannot convert the disks to GPT after installing server clusters.
This is particularly problematic because Windows XP 64-Bit Edition initializes all disks as GPT disks by default. A related problem is that troubleshooting tools designed for MBR (Master Boot Record) disks will not work on GPT disks.
This is related to the way GPT disks are organized. Unlike MBR disks, GPT disks no not rely on a single record (the MBR) to store formatting information on the disk. GPT disks maintain a second copy of their information. The GPT format and locations for this and other specialized partitions (such as manufacturer-installed partitions) are different from their MBR equivalents.
To fix the problem, Microsoft recommends removing the cluster service and converting all the disks back to conventional MBR disks. An Itanium Windows system will support MBR disks, but it needs at least one GPT partition for the operating system.
For more information:
Checklist: Five steps to maximizing disk capacity
Topics: Primary storage
About the author: Jon Oltsik is a senior analyst and storage industry veteran at the Enterprise Strategy Group, focused on information security.