High Sierra Locks Me In

Hi. As I've always done with OS upgrades, before I said Yes to High Sierra, I cloned my boot drive to an external La Cie Porsche SSD, then did the upgrade, which I found tortuous for the first time ever. Now I have everything working. However, although my cloned Sierra drive is seen in Startup Disk, a restart kicks me into the usual black screen, the keyboard lights extinguish, but the cursor is still on-screen. And the Mac doesn't start up.

I tried using Safe Mode - didn't work either. Holding down Option at startup shows me only the internal drive.

I need to use Sierra because some applications essential to my work are not being updated for High Sierra. I'm using a June 2017 MacBook Pro.

So it seems that installing High Sierra on my machine means that none of my past OS boot drives will work. There's no warning of this on the App Store page for High Sierra. That's called a lock-in where I come from. And it's illegal.

Any comments, folks?

Namaste,

David.