This is how you Extend Cluster Shared Volume (CSV)

1. Expand the LUN used to CSV
2. Find the CSV owner
3. Login local on the CSV owner node/host, and run diskpart from Command prompt
4. Type “rescan”
5. Type “list volume”
6. Type “select volume [Disk Id of the CSV volume]
7. Type “extend”

To check the size has extended type “list volume” to see that the volume size now has extended.

Microsoft kb:http://support.microsoft.com/kb/304736