After moving my computer, a Mac Pro 2013 running Sierra 10.12.6, (from one building to another, with a week or so spent in a garage), I found that it would not boot. The system drive was the internal SSD. While I was placing the computer in to a cabinet, the cylinder was unlocked and the computer dropped out and fell a few inches, and fell on to it's side. I have to assume this jostled something important enough to not boot.
I would get the Apple logo, then the circle with a line through it, then, eventually, it booted in to a utility mode with a menu to restore from Time Machine, use Disk Utility, etc.
As I understand it, that's the "Recovery Partition".
Now, I have no memory of ever creating, managing, or maintaining such a thing, but there it was.
Using the Disk Utility, I could not find the native SSD built in to the Mac Pro, but it saw my external SSDs, including my Time Machine drive.
I repartitioned one of the SSDs in to a big enough slice, and restored from Time Machine in to that new partition, and the system came back up.
So, my question is where is the Recovery Partition? I have to assume it's not on the internal SSD -- hard to imagine that simply a partition of the SSD is unmountable, vs the entire card. Is it on my Time Machine drive? That's my guess, but I don't know how to make sure.
At some level, I don't care. "It just worked" and did it's job. But I would feel better knowing that it's not on the internal SSD. My hope is that I can pop that out and push it back and it'll just work, or, barring that, I can simply replace it and nothing more untoward has happened to the machine. As it stands, it seems to be working fine save not being able to see the internal SSD.