I have tried twice now to install Public Beta 3 on my MacBook Pro mid-2010, and both times I got that same error and message after the "restart to install". Then I'm asked to click Restart and I'm back to where I was (Public Beta 2 level). I can't figure out any fix, except waiting a day a or two and trying again. Edit: Tried third time: download from App Store as usual, but then Restart from the Apple menu instead of the Update procedure. The Software Update install started and now I have build 15C40a. However, this Public Beta 3 does not show up in the App Store update list, not as installed and not as available. I'm having a hunch that there is a problem with App Store.