Until 2012, all of the previous versions of MS SQL kept the user info in the master DB. If you restore a DB to another server, those User IDs will not exist in the 'Restored to' master DB. (Even though the USERS are restored with the DB, they do not have a matching row in the master DB and will not work.) The SA user has to remove the old IDs and create new ones, before any user can access it.