I was on 10.11.3 and everything was fine except Spotify causing kernel panics. So I thought I'd update to the latest version of El Capitan. After the apple logo and loading bar, the screen goes black. The white spinning icon appears for a few seconds and disappears and then the cursor appears for a few seconds and then disappears. This just repeats over and over. Booting into safe mode does work and I can log in. Apparently others have had this problem. And a simple solution was found. https://discussions.apple.com/message/30612910#message30612910 So I put my machine in target disk mode and used another computer to delete the recommended folders. Doesn't work for me, same result. So I created an external boot drive - a clean install of 10.11.6 on a freshly formatted drive. It boots fine when I try it with a MacBook. But when I try it on my Mac Pro, I get the same result: black screen, spinning white icon. My Mac Pro won't boot with a fresh install of 10.11.6! So now I'm updating the external boot drive to Sierra to see if my Mac Pro will boot that. At the same time, I don't upgrade my OS until usually a year after release because I like all the major bugs to be ironed out and all my app compatible. So I really don't want to update my main machine to Sierra. Any ideas on how I can boot my Mac Pro 5,1 with 10.11.6? I tried all the usual stuff like repair permissions, reset PRAM, reset SMC. Mac Pro 5,1 GTX 780 flashed by MacVidCards (could that be the problem?) 32GB Ram Thanks for any feedback or ideas.