MacOS に VirtualBox をインストールする際の問題の概要

Warning: Cask 'virtualbox' is already installed.

Failed to unload org.virtualbox.kext.VBoxDrv - (libkern/kext) kext is in use or retained (cannot unload).

Warning! Found the following active VirtualBox processes:

kill 234

~ 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'
...