Ok, so after a little fiddling around, I have found the solution.
I took my HDD to work and hooked it in to an old Win XP machine that I have laying in my office (one of several x.x). I noticed that the disk loaded up with no problems on this machine, so I ran "Chkdsk :H /F" from the command line with Admin privileges.
The command completed successfully after about 15 minutes or so.
Once that was done, I ran a disk cleanup on the drive and found a couple million KB of old files (Recycle bin, old data files, etc). I cleaned everything out, and then hooked the drive back into my Win 8 machine and presto! It loaded right up with no errors or warnings.
I will monitor it for a few days to see if something else comes up, if not, Ill...