I got the following error with Beta 2.
I do not know what is relevant, but the project back-targets .NET 3.5 and is a project that includes T4 preprocessed templates. Neither of these should matter in relation to this error:
Error 4 Unable to write to output file ‘[[full project diretory]]/\obj\x86\Debug\T4CompiledTemplates.pdb': Unspecified error T4CompiledTemplates
(with [full project directory] replaced with the actual directory.
I tried the obvious – unload the project, unload VS, restart the system. None worked.
The solution for me was to create a brand new project (with new name and directory) copy the files, reset the references, etc.