VirtualBox 中很多功能需要安装增强功能，在使用过程，安装增强功能会报下面的错误：
Unable to insert the virtual optical disk /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso into the machine VirtualUbuntu.
Would you like to try to force insertion of this disk?
Could not mount the media/drive '/Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso' (VERR_PDM_MEDIA_LOCKED).
Result Code:
NS_ERROR_FAILURE (0x80004005)
Component:
Console
Interface:
IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}
Callee:
IMachine {480cf695-2d8d-4256-9c7c-cce4184fa048}
解决方案：
在安装虚拟机中打开 terminal，执行下面的命令：
sudo apt-get install virtualbox-guest-utils
参考地址：
https://askubuntu.com/questions/573596/unable-to-install-guest-additions-cd-image-on-virtual-box
