Now Jim... just hold off on that for a bit there... it turned into a quote when I edited it to include the last bit:
"OH... I did boot into Recovery Mode and ran the Hardware Test. It all passed... all OK. The Test says that if your hardware is OK then it's software and I can see that. I have disconnected the external right now... later I'll reconnect it and run the test again."
My only sad excuse for myself is that it was very late, (or very early, depending on how you see it), and it just was beyond the by poor battered brain at the time... I shall try to fix it to look better now!
Should I have a problem I promise to promptly send you a message for help!...