Which drive have you selected as your boot drive SSD or HDD? Go into Preferences and check your Startup Disk setting.
Sadly, Intel had some issues with the PCH logic (Intel HM77 (SLJ8C) platform controller hub) within this system. Depending on when you got it the first models had an issue with the SATA ports. Here's a bit more:
[https://www.anandtech.com/show/4142/intel-discovers-bug-in-6series-chipset-begins-recall|Intel Discovers Bug in 6-Series Chipset] & [https://www.anandtech.com/show/4143/the-source-of-intels-cougar-point-sata-bug|The Source of Intels Cougar Point SATA Bug] This is not to say this is your issue but it would be wise to check.