UPDATE/WORKAROUND: I was so frustrated by this yesterday that I gave up and went hiking. At the trail head I met two cute girls in bikinis. The second was a Mac person and kind of vague. The first suggested that I try connecting my SD card directly to my computer via a USB adapter. After our hike, I went home and tried this. The 64GB (a SanDisk Ultra SDXC) was recognized by Windows (XP SP3, for which I refuse to apologize), but Windows told me that this exFAT "drive" needed to be formatted. I read that OS's prior to Vista need a patch to read SDXC cards, but when I tried to download the patch from Microsoft I got a message saying that the patch was older than my OS, meaning I suppose prior to SP3. However I also noted that there have been issues with some versions of Vista not being able to read SDXC cards. BTW I tried several different SD card to USB adapters, although the ones I have are all passive devices.
So, in further desperation, I then tried my old SD card from my old phone. This card is a 32GB Samsung Evo UHS-1. My computer instantly recognized this card and its contents -- no problem. I then went into Media Center and in Sync Handheld Options designated this card as a new device. This worked perfectly. MC asked me if I wanted to recognize the data that was already on the card, and I said yes. Then I set all of my other Sync Handheld options (DSP, Conversion, etc.) and ran Sync with no problems whatsoever.
After I did this I installed the 32GB SD in my Galaxy S4 (which, of course, is an Android phone). It took Neutron a little while to find and "load" all of the "songs," but after it did everything worked perfectly. I decided to use the "Normal" OGG Vorbis setting (which results in a bitrate of around 156) but also to convert (compress) for high bitrates.
So I have solved my problem, albeit partially, while leaving several questions unresolved. One could throw the whole thing back onto my XP fanaticism, but bear in mind that I was able to manually (meaning using Windows and not MC) copy files onto the 64GS SDXC card via the Android function, and then play these using Neutron. So nothing either in XP or in the Android is stopping me from putting music files onto my 64GB SD.
Personally I would suggest that a fully functional and highly customizable Sync Handheld function should be a very high priority for MC21. When it works it is absolutely fantastic.
Final comment: In general I would like more "stats" in MC. Maybe they are there but if they are I can't find them. By "stats" I mean, for example, the number of tracks (songs) in a playlist and how many bytes that represents.
Anyway, any further enlightenment on this topic would be greatly appreciated. And if anyone knows of an external card reader (preferably USB-3) that will allow my XP SP3 computer to deal with my 64GB SDXC card, please let me know!