I was restoring a database backup in sql server after a long time, and hit upon this error. Well how difficult can restoring a db backup be in sql server express.
Hmm, not that difficult, here is what happened.
System.Data.SqlClient.SqlError: The operating system returned the error '5(Access is denied.)' while attempting 'RestoreContainer::ValidateTargetForCreation' on 'C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\MDS.mdf'. (Microsoft.SqlServer.SmoExtended)
The backup creator had MSSql version 10 installed, so when he took the backup it also stores the original file path (to be able to restore it in same location), but I had version 11, so it could not find the destination directory.
So I changed the output file directory to C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\, and it was able to restore the database successfully.