To let ASP.NET to search the database location, use the following connection strings in the web.config in specific cases

-> in the default data folder

connectionString="Data Source=.\SQLExpress;Integrated Security=True;User Instance=True;AttachDBFilename=|DataDirectory|aspnetdb.mdf"

-> in an absolute path

connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\NewMembershipProvider\App_Data\Members.mdb;Persist Security Info=False"