I posted another thread about migrating from the 2013 to the 2015 iMac. I have now completed that successfully, and have a couple questions going forward. First, I notice that when the 2015 machine starts up, the Apple logo/progress bar screen is present until the bar is only about half way, then the screen goes black, and then it flashes very briefly again right before the desktop comes in, and then it flashes a Third time as the desktop finishes settling in (desktop doesn't disappear, it's just like a brief flash cross-fade image of the Apple logo screen). Is that okay/normal? Secondly, I need to erase my 2013 machine before selling it, but the new Disk Utility has no zero-out option. There must be a way to erase the hard drive. Has anyone figured it out yet? Thanks!