When you build a Microsoft cluster with Windows Server 2003 SP1 nodes, mount points are fully supported. This is useful on larger clusters because you can save on drive letters.

For example, if you create an A/A/P/P cluster for Exchange 2003 where each active node has four storage groups you easily end up with >16 drive letters. One drive letter for each storage group's databases (8), one drive letter for each storage group's logs (8) and then some drive letters for quorom, SMTP queue drives, and so on. Still not a problem but you see what it leads to when you add more nodes.

To avoid using this many drive letters, you can just mount some drives in a folder of another drive. For example, if you use G:\ for Exchange databases, you could use G:\Logs for the log files. In this case, a separate volume is mounted on that folder.

This worked very well in Windows Server 2003 (without service pack) but with service pack 1, there are some issues. The issues are descibed in http://support.microsoft.com/kb/898790 and there is a patch that solves it.