I had something interesting happen yesterday. I went to turn on my late 2013 27" iMac and it would not boot. I got the Apple logo and the spinning icon but it would just stay on that screen and not do anything. I tried all of the normal stuff. Unplug all peripherals, reset NVRAM, repaired permissions, repaired disc and checked hardware (cmd-d). All came out fine. Obviously the hard drive (1 TB Fusion) and all of the hardware was working so it had to be a software issue. So as a last ditch effort, I went into the recovery partion and re-installed Mavericks (leaving the data in place) and Voila! The iMac booted up perfectly and is running better than ever. My question. Usually I keep things pretty clean on this iMac. Occasionally I will empty the trash, repair permission and check the interal HD. I noticed that when I re-installed Mavericks, I gained about 30GB worth of hard drive space. Is there a cache somewhere in this OS that also needs to be cleaned that might have caused this issue? I've been using iMacs since 2006 and this is the first time this has happened. Really weird. Thanks.