问题一：Cask ‘virtualbox’ is already installed.
如果不是通过Homebrew安装的virtualbox，在使用brew cask install virtualbox安装virtualbox的的时候，会提示：
Warning: Cask 'virtualbox' is already installed.
那么就需要重新安装virtualbox，命令：
brew cask reinstall virtualbox
如果还不巧，你的virtualbox刚好在运行中，又会提示：
Failed to unload org.virtualbox.kext.VBoxDrv - (libkern/kext) kext is in use or retained (cannot unload).
那么，就需要网上拉日志，找到：
Warning! Found the following active VirtualBox processes:
这些是运行中的虚拟主机列表，其中第一列是它们的 ID，用kill命令关闭服务，如：
kill 234
然后再重新安装：
brew cask reinstall virtualbox
问题二：Error: Failure while executing;
安装virtualbox的最后一步会要求输入电脑的登陆密码，不过在你输入密码后，可能还是会提示错误，这是因为权限的问题，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' ...
这主要是因为安装源被block了，电脑打开：System Preferences → Security & Privacy → General，然后点右下方的“Allow”允许，再重新安装。