Mark, As dynamox pointed out converting a basic disk to a GPT disk can only be done with an empty disk before any partitions have been created. Unfortunately you will either need to backup the 2TB LUN and restore the data to a 3TB LUN or add another LUN to the cluster. You use diskpart.exe to convert a disk to GPT.  Also on a Windows 2003 cluster you will need a hotfix to use GPT LUNs over 2TB: [URL]
Hi, I have a windows server cluster (win 2003 storage server 32 bit service pack 2) that has a Clariion Metalun presented. The metalun is currently 2 * 1000gb luns to produce a 2tb lun at the OS (concatonated at the clariion CX310). The lun is a Basic Disk and managed by MSCS.  I have been asked to increase the size of this disk so that it will be 3tb. Is this possible?  Can i increase the Clariion cx3-10 metalun by another 1TB so that is is 3TB if total? Can i then increase the Windows 2003 disk so that is is 3TB keeping in mind that the disk is clustered? I read that a microsoft patch may be required to conver the disk to GPT since the server is clustered.  I read that to increase the disk to greater than 2TB it must be a GPT disk at the windows level. Can i convert to GPT if there is already data on the disk? I read than i cannot do this if data is on the disk.  Thanks for any help.  Mark Wragge
metalun to create disk greater than 2tb on windows