This issue occurs when you try to regenerate a 3 way mirror in Windows Server 2012 R2 and Windows 8.1.
To resolve this issue, install update 2955164. For more information about how to obtain update 2955164, click the following article number to view the article in the Microsoft Knowledge Base:
2955164 Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 update rollup: May 2014
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updatesThis issue occurs because the child packets raid1 builds for regeneration do not have the _CopiesMask value set. Therefore, raid1 never picks another copy even when one is available.