錯誤
E: Failed to fetch cdrom://Xubuntu 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.
方法：
1. root 下
gedit /etc/apt/sources.list
2. 打開文件後將這一行給註釋掉
deb cdrom:[Xubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main multiverse restricted universe
3.現在進行sudo apt -get update 就可以了