Okay - while this doesn't really tell me what it was, or why it showed up...I DID find a way to fix it.
Before you try this, remember that it was only my current install of Katya that couldn't access the partition - all other LiveCD/installed OSs, etc could access the partition in question without problem. Also, I was working on a single extended partition with one logical partition in it. This should work for multiple partitions, but you'd have to write down a lot more info before starting. Most importantly - BACKUP ALL YOUR DATA!!!!
Here's what I ended up doing:
Backed up my data using Windows, then booted back into Mint.
sudo fdisk -l -u /dev/sdd (/dev/sdd was the drive where the problem partition was)
Among other things, it showed me that /dev/sdd2 was my extended partion, and that it began at 204796620 and ended at 675838484.
It also showed me that the logical partition was at /dev/sdd5 and started at 204796683 and ended at 675838484.
So, I wrote the starting and stopping sectors down for the extended partition and logical partition.
I then did sudo fdisk -u /dev/sdd (not sure if the -u does anything, afterall, I'm new)
Once in fdisk, I pressed c to disable ms-dos compatible (I was prompted to do so automatically), you need the units to display as sectors, so only press u if you need to change the displayed units.
I listed the partitions by pressing p (just to make sure I was on the right disk, and that everything matched my earlier listing)
Then, I pressed d and chose partition 5 to delete. (to delete my old /dev/sdd5 logical partition)
Then, I pressed d and chose partition 2 to delete. (to delete my old /dev/sdd2 extended partition)
I then wrote the changes by pressing w (Make sure to read whether or not the system needs a reboot at this point - if so, do it).
After reboot, I came back and sudo fdisk -u -l /dev/sdd again - just to make sure the partition wasn't listed.
I then sudo fdisk -u /dev/sdd
I pressed c again when prompted.
I again pressed p to list the partitions, and found the space listed as unallocated...perfect!
Then, (this is the important part), I selected n to create a new partition, but instead of accepting the default sector starting point, I entered the old LOGICAL partition starting point (in the above example this was 204796683 - the beginning sector of my old /dev/sdd5 partition which contained the data) for the beginning sector, and 67583884 for the ending sector (which was the ending sector from my original /dev/sdd5).
I then changed the partition id type to 07 (NTFS) to match the original /dev/sdd5 partition.
I pressed w to write the changes.
I rebooted the system and some of my files were back and working! (But not all of them)
I rebooted into Windows and it wanted to run scandisk - which I allowed. At completion, all my files were there, and the partition was even named with the original partition name (I didn't retype this or anything).
I booted into Katya again and lo and behold - everything works like a dream again!! I can mount my music partition!!! Life is good!
Marking as solved, but I'll try to remember to check back every once in a while in case I can offer any clarifications.
Thanks for all of those who read through my original posts - I appreciate the effort!