Q. What is disk quorum with Storage Spaces?
A. When thinking of quorum, you often think of nodes in a Windows cluster. However, disks in a Storage Space also use quorum, and the requirement is that more than 50% of the disks must be present to make quorum and make the pool available.
This is the same logic as nodes in a cluster and is designed to ensure in the event of some kind of break you would never have more than one set of disks making quorum and offering storage.