I had a terrible time trying to install previous Betas - invariably it went wrong at some stage and I needed to re-install everything from scratch. This time it worked. I did two things differently: I made sure there were no kernel extensions installed. This meant removing the SOPHOS antivirus program I had to install for work. I booted holding Shift (whether this actually does use a "safe mode" or is a superstition I don't know any more :-) The installation of the Beta then went smoothly. Good luck!