I'm on a 2011 Mac Mini running El Capitan. So I downloaded macOS Sierra from the App Store. I run the installer and get to the screen where it asks you to choose the destination disk for install. It lists my external HDD and my second internal HDD as options, and grayed out are my Time Machine backup disk, and the Macintosh HD (an SSD). When I click on the grayed-out Macintosh HD, a popup says: "This disk has S.M.A.R.T. errors. This disk has a hardware problem that can't be repaired. Back up as much of the data as possible and replace the disk." My Macintosh HD is a 480GB OWC Mercury Electra 6G SSD. I've had no issues with it since I had it put in back in January 2015. I've never had a message like this come up previously. I just ran First Aid in Disk Utility while in Recovery HD and it's come back fine, says the disk checks out. And I had no issues when I did the last upgrade from Yosemite to El Capitan. So I'm having a hard time figuring out why the hell I can't even get started with installing Sierra. If anybody's got any insight into this issue, has seen it also, or can give me any advice on where to go from here, it'd be appreciated.