Help! Unable to boot from any disk after Sierra installation

Hi all,

So, I finally upgraded to Sierra and everything was fine the first couple of hours. I closed the lid only to realise 3 hours later that the mac never went to sleep but was instead running the fan full throttle. I had to do a hard reboot and to my horror saw that the computer stalled at the white start up screen (with the apple logo).

Additional info that might be relevant: A couple of years ago, I replaced the CD-drive with a SSD drive for applications and OS. I still have the original internal HD installed as well (but without OS of course). I have installed new 8GB RAM as well.

So far I've tried:

- Disk Utility in recovery mode (claims all disks are fine)

- Safe boot (still stuck at white screen)

- Reset NVRAM

- Reset SMC

- Single user file consistency check (fsck -fy) – showed some errors first time, but after running again it claims that everything's fine.

- Sierra reinstall three times (but of course the download itself might be corrupted)

- Booted from an external drive with a backup partition. Guess what? Stuck at white screen.

When I boot the computer in safe + verbose mode, there are a couple of errors that might interest people who know more than I do: the picture of the screen is attached below. The "master-entitled process" that was cropped off the image is named 'launchd' (1).Verbose photo

I have no idea whether this is physical damage, boot sector confusion or something else? Please help.