http://blogs.technet.com/mbaher/archive/2010/02/15/kb978790-update-to-windows-2008-file-share-witness-failure-state.aspx

KB978790 – Update to Windows 2008 File Share Witness failure state
The platform team released an update to the Windows 2008 FSW that affects the cluster state of the FSW is in a failed state and come back online.

Consider the following scenario:

•You configure a Windows Server 2008 failover cluster to use a Node and File Share Majority quorum model.
•The File Share Witness directory is not available. This behavior causes the File Share Witness resource to enter a failed state.
•The File Share Witness directory becomes available later.
In this scenario, the File Share Witness resource remains in a failed state even though the File Share Witness directory is available. Additionally, when the File Share Witness resource is in a failed state but is needed to maintain the quorum, the cluster loses the quorum.
Note You expect the quorum to be maintained when the File Share Witness directory is available

The hotfix can be downloaded from here http://support.microsoft.com/default.aspx/kb/978790