Not entirely sure if I should've posted this in a macOS section or this one; my issue involves both I think. I own a mid-2014 retina MacBook Pro. For a long time, it was running Sierra on one partition, and Windows 7 via Bootcamp on another. Both were working fine. About a week ago, I made a USB installer for OS X Mavericks 10.9.5 that I wanted to use for a fresh OS X installation over Sierra. (only because I like the skeomorphic aesthetic of OS X prior to yosemite. I knew it would be compatible, because my MBP version was released before yosemite.) Before erasing my Sierra volume, I made a time machine backup of it on an external drive. I also installed the Sierra OS on the drive using the time machine backup. So I basically made an external carbon copy of my Sierra volume. I then installed Mavericks over my internal Sierra volume. Everything was working fine; I was using mavericks for about a week, installed updates, apps, etc. It was also noticably faster than Sierra was. Yesterday, I shut down my laptop for the first time to use the bootcamp partition (which still works, by the way. I'm using it to post this). However, when I boot it back into Mavericks, it gets endlessly stuck on the apple logo boot screen w/ a spinning gear. I don't understand why it won't boot now, when beforehand it was working fine upon a fresh install.