It seems that if you have an old (not sure how old, but not current) version of VirtualBox on your OS X 10.11 beta <6, you'll not be able to complete the installation of OS X 10.11 beta 6 due to a kext induced crash during the late stages of the install migration.
There are other threads that have appeared with discussions of these issues - e.g. here: https://forums.developer.apple.com/thread/13245.
Finding a fix for this issue is not easy - and several have resorted to fully restoring a working OS X 10.11 beta 5 or earlier system, uninstalling the VirtualBox app using an uninstaller, and then trying to migrate to Beta 6 again.
I found an easier way to fix this issue on my system. This is what I did:
- I restarted my system in safe mode - hold down the shift key during the boot sequence. N.B. on my system this resulted in a very long pause in the resart sequence, and then once the restart had begun the completion of the Beta 6 install restarted, and this also took a long time (and required me to re-log-in twice). But it did get me to a working desktop eventually.
- In safe mode I then opened safari and downloaded the VirtualBox uninstaller from here - http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Installer/darwin/DiskImage/VirtualBox_Uninstall.tool
- I moved the uninstaller script to my home folder (~)
- I ran the uninstaller in terminal using this command - sudo /bin/sh VirtualBox_Uninstall.tool Intially this command did not work, but adding the /bin/sh to the command line instruction made it work OK eventually. This is the terminal log of what happened:
sudo /bin/sh VirtualBox_Uninstall.tool Welcome to the VirtualBox uninstaller script. The following files and directories (bundles) will be removed: /usr/bin/VirtualBox /usr/bin/VBoxManage /usr/bin/VBoxVRDP /usr/bin/VBoxHeadless /usr/bin/vboxwebsrv /usr/bin/VBoxBalloonCtrl /Library/StartupItems/VirtualBox/ /Library/Extensions/VBoxDrv.kext/ /Library/Extensions/VBoxUSB.kext/ /Library/Extensions/VBoxNetFlt.kext/ /Library/Extensions/VBoxNetAdp.kext/ Do you wish to uninstall VirtualBox (Yes/No)? Yes The uninstallation processes requires administrative privileges because some of the installed files cannot be removed by a normal user. You may be prompted for your password now... Successfully unloaded VirtualBox kernel extensions. Done.
- I then restarted the machine and let it run normally - the restart took a very long time (it paused at the end of the grey-line phase for ages) - but eventually I got back to OS X 10.11 beta 6.
This method worked for me, but maybe not anyone else. If it does work for others, this might be a good thing compared to restoring an old version of OS X from Time Machine.