I've been having this issue since USB 3 pretty much first came out. My 2012 and 2014 Mac Mini's both experience frequent USB 3 drive disk ejections. It doesn't matter if my computer is on or sleeping, the drive will eventually eject - it could be 15-20 minutes or 8-10 hours. It seems to happen randomly and sometimes will be fine for an entire day or more. The external drive has its own power supply that I use. When I notice the ejects, the drive seems to remain powered on. I've replaced the USB 3 cable. I've tried different ports. Clean install of OS X 10.9, 10.10, etc. I need an external drive connected as my iTunes library is on there so I can stream content to Apple TV. I disabled putting drives to sleep in System Preferences as well hoping that would fix it. Any idea what's going on? What else can I try? I've even left it without iTunes running to see if it was iTunes doing something. At this point, I'd like to switch over and give TB a try except it seems to be a very pricey alternative and not many "BYOD" options out there. I prefer RAID 1, but a single drive wouldn't be bad as long as I can store 2TB on it.