시스템 업데이트시 cdrom:// ~~ 오류가 아래와 같이 나올 때

설치한뒤 업데이트 하면 아래와 같이 나올때

W: The repository 'cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1) xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)/dists/xenial/main/binary-amd64/Packages  Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
E: Some index files failed to download. They have been ignored, or old ones used instead.

시스템설정에서  

소프트웨어 및 업데이트에서 

맨밑에 CD-ROM/DVD에서 설치 체크표시를 제외하고 다시실행하면 문제해결