问题描述：
Ubuntu中，使用sudo apt-get update更新，会出现如下错误：
W: Failed to fetch cdrom://Ubuntu-Server 15.10 _Wily Werewolf_ - Release amd64 (20151021)/dists/wily/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
W: Failed to fetch cdrom://Ubuntu-Server 15.10 _Wily Werewolf_ - Release amd64 (20151021)/dists/wily/restricted/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
W: Failed to fetch cdrom://Ubuntu-Server 15.10 _Wily Werewolf_ - Release amd64 (20151021)/dists/wily/main/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
W: Failed to fetch cdrom://Ubuntu-Server 15.10 _Wily Werewolf_ - Release amd64 (20151021)/dists/wily/restricted/binary-i386/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.
问题原因：
开启了cdrom源，系统会加载cdrom源进行更新
解决方案：
进入/etc/apt/sources.list目录，备份sources.list文件，修改sources.list文件
cd /etc/apt/
cp sources.list sources.list.origin
vim sources.list
# 注释如下内容
# deb cdrom:[Ubuntu-Server 15.10 _Wily Werewolf_ - Release amd64 (20151021)]/ wily main restricted