Are you equipped to remove the internal HDD and put it in an external enclosure? If so, I'd

1. Put internal 500GB HDD in external closure.
2. Boot from your 1TB HDD.
3. Use Disk Utility to reformat the formerly internal 500GB HDD now in the external closure.

I truly don't know that this procedure will help. I offer it as another way to reconfigure the items you may have available to you. "Little Ruckus" may remain unmountable whether it's internal or external.

To be honest, I'd likely just replace the HDD with a new one, though I can appreciate that, given your current location, it may not be possible.

Well looks like we have collectively hit a wall here. Maybe someone could speculate on what could be making the internal drive too busy to un-mount even after multiple re-boots to other/external drives with current system software running. I can't figure out how a rogue file or process could start up so consistently that it can't be interrupted in some way (either during spin-up or by doing some kind of general kill command via the Terminal). This is all way above my pay grade, but I'm a good learner and I am now just curious what the heck is going on here more than anything else.

Is it possible that it is a hardware problem that just makes it look like it is busy, even though it passes a repair process with Disk Utility? I dunno, it's driving me nuts.

Yeah, I don't really have a place to get this done that I know of. I am going to look around some of the shops here and see if I can dig up a firewire cable or an enclosure to see if I can spark it up as you suggest. I'll let you know ;)

"Is it possible that it is a hardware problem that just makes it look like it is busy, even though it passes a repair process with Disk Utility? I dunno, it's driving me nuts."

Failing HDDs are quirky. While what you report seems to be the kind of thing that can be fixed with software, the fact that software isn't fixing the trouble makes me wonder about a hardware issue. In the US, a new 500GB HDD is around $60--a small price to pay to preserve sanity and get back to work. You'd pay at least $20 in the US for a cheap USB enclosure to hold and continue testing the old internal, so you may just want to replace the HDD and move on.

You say it's a "Macbook Pro 2012." Open System Profiler (Apple menu > About this Mac > More Info . . . ). What's the Model Identifier? It will be something along the lines of "MacBookPro9,1." Based on the fact that it has a HDD rather than an SSD, I'm guessing you have the "mid 2012" MBP (HDD) rather than the "mid 2012" Retina MBP (SSD).

To replace or remove the HDD isn't difficult. Instructions and video here for 13" non-Retina mid 2012 MBP: https://www.ifixit.com/Guide/MacBook...lacement/10378. Here for the 15": https://www.ifixit.com/Guide/MacBook...lacement/10761.

Repairing anything when far from home often reminds me of the "square peg in round hole" scene from Apollo 13 as ground control tries to jury-rig a solution to fix the carbon dioxide scrubbers:

"We gotta find a way to make this [cube] fit into the hole for this [cylinder] using nothing but that [pile of items available on-board Apollo 13]."

http://www.youtube.com/watch?v=C2YZnTL596Q

Sorry - I don't understand why you are saying that there are "no error messages" when I see on your screen shot that it says "Can't repair ..." in red.

I have no idea why the (internal) drive would be busy.
But one thing to try is to make sure that Spotlight is not trying to index that drive.
Go into Spotlight preferences (after booting from the external drive) and make sure that the internal drive is excluded from Spotlight.
Or maybe disable Spotlight entirely - google for instructions on doing that.