MSSQL Error 'The underlying provider failed on Open'

Question

I was using an .mdf for connecting to a database and entityClient. Now I want to change the connection string so that there will be no

Answer 1

I found the problem was that I had the server path within the connection string in one of these variants:

SERVER\SQLEXPRESS
SERVER

When really I should have:

.\SQLEXPRESS

For some reason I got the error whenever it had difficulty locating the instance of SQL.