diskpart WIndows 2003 cluster and EMC SAN

From http://support.microsoft.com/kb/300415/en-us
extend [size=n][noerr]

Use the extend command to cause the current in-focus volume to be extended into contiguous unallocated space. The unallocated space must follow (it must be of higher sector offset than) the in-focus partition. The intended use of this command is to grow an existing basic data partition into newly created space on an extended hardware Raid logical unit number (LUN).

If the partition had been previously formatted with the NTFS file system, the file system is automatically extended to occupy the larger partition, and data loss does not occur. If the partition had been previously formatted with any file system format other than NTFS, the command is unsuccessful and does not change the partition.
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So basically MS are using the words volume and partition unterchangably. It is possible for diskpart to succeed in extending the partition but not the volume - then you see no space gain under My Computer but you see a bigger partition under disk manager. You cannot however chose to extend the partition and then the volume, it's a single command that does both.

If it does fail there's a program called fsextend.exe that will extend the filesystem (volume) that MS used to email you but there's a hidden option under diskpart now. See http://support.microsoft.com/kb/832316/en-us for this but it only happens when the server is busy doing other things, since you're taking the cluster offline to do the expansion it shouldn't happen.