問題一：Cask 'virtualbox' is already installed.
如果不是通過Homebrew安裝的virtualbox，在使用brew cask install virtualbox安裝virtualbox的的時候，會提示：
Warning: Cask 'virtualbox' is already installed.
那么就需要重新安裝virtualbox，命令：
brew cask reinstall virtualbox
如果還不巧，你的virtualbox剛好在運行中，又會提示：
Failed to unload org.virtualbox.kext.VBoxDrv - (libkern/kext) kext is in use or retained (cannot unload).
那么，就需要網上拉日志，找到：
Warning! Found the following active VirtualBox processes:
這些是運行中的虛擬主機列表，其中第一列是它們的 ID，用kill命令關閉服務，如：
kill 234
然后再重新安裝：
brew cask reinstall virtualbox
問題二：Error: Failure while executing;
安裝virtualbox的最后一步會要求輸入電腦的登陸密碼，不過在你輸入密碼后，可能還是會提示錯誤，這是因為權限的問題，主要信息如下：
~ brew cask reinstall virtualbox
==> Caveats
To install and/or use virtualbox you may need to enable its kernel extension in: System Preferences → Security & Privacy → General ... ==> Running installer for virtualbox; your password may be necessary. ==> Package installers may write to any location; options such as --appdir are ignored. installer: Package name is Oracle VM VirtualBox installer: Installing at base path / installer: The install failed (The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.) ==> Purging files for version 6.0.10,132072 of Cask virtualbox Error: Failure while executing; `/usr/bin/sudo -E -- env LOGNAME=gary USER=gary USERNAME=gary /usr/sbin/installer -pkg /usr/local/Caskroom/virtualbox/6.0.10,132072/VirtualBox.pkg -target /` exited with 1. Here's the output: installer: Package name is Oracle VM VirtualBox installer: Installing at base path / installer: The install failed (The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.) Follow the instructions here: https://github.com/Homebrew/homebrew-cask#reporting-bugs /usr/local/Homebrew/Library/Homebrew/system_command.rb:111:in `assert_success' ...
這主要是因為安裝源被block了，電腦打開：System Preferences → Security & Privacy → General，然后點右下方的“Allow”允許，再重新安裝。