VirtualBox 中很多功能需要安裝增強功能，在使用過程，安裝增強功能會報下面的錯誤：
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}
解決方案：
在安裝虛擬機中打開 terminal，執行下面的命令：
sudo apt-get install virtualbox-guest-utils
參考地址：
https://askubuntu.com/questions/573596/unable-to-install-guest-additions-cd-image-on-virtual-box
