SharePoint 2013: NET 3.5 framework add feature error - the source files could not be found

Problem

The .NET 3.5 Framework feature can not be added to a new Windows Server 2012 build.  This problem is encountered during the build of a new SharePoint 2013 web front end (WFE):

No matter how the source disk is made available to the Add Feature wizard, whether using different network drives, mounting it locally, copying it to the local drive, etc., the error message is the same.  A similar message is encountered when attempting to add the .NET 3.5 feature via PowerShell script.  This is the first time you have encountered such an error building a new Windows Server 2012 instance for SharePoint 2013. You last successfully created a new Windows Server 2012 instance in July 2014.

Solution

1. Uninstall KB2966827.
2. Reboot.
3. Add the .NET 3.5 Framework feature and all its subfeatures.
4. Reboot
5. Re-install KB2966827.

References

• [Fix] 0x800F0906 and 0x800F081F Error Messages While Installing .NET Framework 3.5 in Windows 8/8.1
• Microsoft Security Bulletin MS14-046 - Important
• MS14-046: Vulnerability in the .NET Framework could allow security feature bypass: August 12, 2014

Notes

• Many thanks to Steve at AskVG and to to my team's lead sysadmin for originally presenting and finding the solution to this vexing problem.
• Steve indicates that three updates are the cause of this problem:
• KB2966826
• KB2966827
• KB2966828
All of these updates are associated with the MS14-046 security update of August 12, 2014.  Only KB2966827 applies to Windows Server 2012.