Glad to note that all your HDDs are ok and it is not a problem with any of those.
But the the problem is now trickier.(atleast for me here.)
As always "there'll always be someone who will know something you (or me
) don't." So let us wait for other experts to dig into this even as I dig for any clues.
In the meanwhile you may post whether you have done anything unusual preceding this event - such as trying to dual boot etc., ( I have only heard of this kind of boot problem trying to install and boot various Linux distros but my knowledge on these is next to nil.
So will not venture into making any guesswork)