My 5TB HD now thinks it's a 3TB?
Well, chalk this one up to pilot error.  It turned out that what I had thought was a single drive in two partitions, wasn't.  I was right that my TM drive was 5TB, but what I had forgotten was that my Media drive, was a completely separate, older 3TB drive that was just somewhat hidden in my media cabinet.  It looks like the TM drive had disappeared due to a cabling issue of some sort.  Unplugging and then plugging back in its USB, caused it to suddenly reappear as expected.  

Oh well, I'm now also the proud owner of a new 8TB drive (which was cheaper than the 5TB drive was 18 months ago!) so I guess I'll look at this as the universe's way of telling me once and for all that I need to stop running out of space on my Media drive.  LOL