what is the purpose of a quorum disk
Whenever a witness is set for a database mirroring session, quorum is required. Quorum is a relationship that exists when two or more server instances in a database mirroring session are connected to each other. Typically, quorum involves three interconnected server instances. When a witness is set, quorum is required to make the database available. Designed for high-safety mode with automatic failover, quorum makes sure that a database is owned by only one partner at a time. If a particular server instance becomes disconnected from a mirroring session, that instance loses quorum.