I have a database file .mdf from MS SQL EXPRESS in folder:
.mdf
C:\\Program Files\\Microsoft SQL Server\\MSSQL10.SQLEXPRESS\\MSSQL\\DATA
The key is "operating system error 5". Microsoft helpfully list the various error codes and values on their site
https://msdn.microsoft.com/en-us/library/windows/desktop/ms681382(v=vs.85).aspx
ERROR_ACCESS_DENIED 5 (0x5) Access is denied.