Thank you for the response.
Just prior to reading it, I poked around with EasyBCD and -- while not the pretty graphical interface it once was -- was able to add back the option for the upgraded 10. ...

No ideas? Anyone?
Short of physically disconnecting Drive 0, I've no idea how to get back to my other OS. (And I'm not keen on the idea of having to install a swappable bay to get around this issue...

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Although it's probably obvious, Disk 0 is the current Clean 10 install, and Disk 2 is the other 10 upgrade install.

Okay, I've clearly managed to mess something up, but here goes...

Prior to a couple days ago, my system was setup in a dual boot configuration, with Windows 8.1 (not set as Default, but...