I have had the same problem.
After moving .edmx file with EF model to separate assembly mentioned error caused my headache: “Unable to load the specified metadata resource”.
EF version 6.1
Solution:
Old value:
metadata=res://*/Data.DataModel.csdl
New value:
metadata=res://*/DataModel.csdl
Initially .edmx was under project’s folder and I have moved it into project’s root.