I am experiencing a reoccurring issue with my Mac Pro 5.1. I purchased this machine last summer. I’m running Sierra 12.12.6, which resides on a Samsung 250EVO SSD, plus I have Sierra installed on another mechanical drive for emergencies. Yesterday I shut down my Mac to do a bit of cleaning. When I started it up it would only boot to the recovery partition, showing the window with Mac OS X Utilities. If I quit Utilities and choose my start-up disk, it does the same thing, boots from the recovery partition. I restarted choosing my back-up version of Sierra on the mechanical drive, it boots fine. I have tried resetting NVRAM, SMC reset, no luck. I have tried swapping the drives to different bays, no luck. The SSD will not boot (only in recovery). It is visible, disk utility reports no errors, reports that it’s bootable and the disk is okay. I tried plugging it in to an iMac via USB. The disk shows up, but when I select it as a startup disk, same thing, only boots in recovery. Additionally, DriveDX and EtreCheck report no problems. I had this same exact problem in November. I shut down the machine for some cleaning, but when I fired it up, same deal, only boots from recovery. This was with a completely different drive (same model though - SSD Samsung 250EVO). What I ended up doing was a fresh install, migrating everything over from the un-bootable drive and all was well, until yesterday. I dread going thru this again. Any ideas? I can’t figure out if I have some funky hardware issue that’s corrupting my system or it’s software related. Any insight would be greatly appreciated.