I upgraded an existing install of Yosemite to ElCap on an old (but supported) Mac Pro. The install appeared to go fine, and it boots up fine to the login screen. Once I log in, I get a permanent loading spinner and nothing shows up. The only time I can get anything to appear is if I receive an iMessage, the Messages app will load and I can interact with it normally. In that scenario, I actually get access to the Apple Menu and can interact with a few basic functions there (like the App Store, or Preview, or iTunes from recent Applications). I cannot interact with Force Quit, Restart, Shut Down, etc. I get the exact same interaction when trying to boot in Safe Mode. Just permanent spinner after user authentication. I get the exact same interaction when logging in as my Admin account or root. I also created a new Admin account after the issue started, and that gets the exact same behavior. I've tried reinstalling. I've tried "First Aid" in disk utility. I've tried resetting NVRAM. I've tried removing the plist and saved states from Finder in case it was a Finder issue with launching. I am completely out of ideas. Anyone have any other troubleshooting tips other than just wiping and starting fresh? It's not a mission critical machine obviously, butI would still rather not.