That's strange, Tom. Your original drive should be exactly as it was, other than changed user files.
What I'd suggest, to start, is turning all the way off, and disconnecting all peripherals except for mouse, keyboard and display. Then, hold down Cmd-Option-v and power on.
That'll log what's going on during the boot process, and -- while technical -- it will hopefully give a message that will help determine what's wrong...
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--Dave Nanian
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