I have SSD drive, on which Win 8.1 is installed, and booted from.
I also have 3 other HDDs connected, not RAID or something.
I have removed one of them, and after next start PC turned off unexpectedly during Windows startup process. I mean, were already was Windows logo on screen.
Then tried and removed another HDD (putting previous one back) ... same result.
Then I put back all HDDs to their place - Windows started up correctly (allowing me to write this now).
What... What is this?
UPDATE
In the end, it's failed PSU. I don't know how is this all related, but I replaced my current one, with older PSU I had, and all worked miraculously.