Joined to core DC. Can't add users from other domains

Did you set the forest to be shared in your domain settings on the readynas this is under the security settings. Also make sure the Time zone is set to the correct time zone in Clock tab. Back to security settings, Also make sure your Domain type is set to ADS. Make sure your NTP points to the IP address instead of hostname or domain name. Make sure your DNS points at your DC also using IP. Also make sure that when your in domain mode that you set your permissions using the your Domain controller with the admin account, set sharing on all folders to everyone - or just dont touch the sharing tab, just play around with the security tab. Remember the more specific rules win.