Application HA clustering has dropped a disk... :-/
After I completed the Application HA clustering of SQL2008 across two Windows 2008 R2 nodes, I found that the SQL installation's virtual Backup disk (M: for reference) remained on the first node after I'd initiated a switch to the second node via the High Availability tab in the vSphere Client. The other disks re-attached to the second node ok.
On closer inspection in Cluster Explorer on one of the nodes I discovered that the Mount Point for that disk was completely missing!
I attempted to manually create the Mount Point and brought the resource online in the cluster on the first node, but attempting the switch operation again failed as the virtual disk failed to re-attach to the second node.
How to fix without blatting the clustering / SQL installations?