The PCM-M10's capacity just jumped to 64GB!
The Sony PCM-M10's user manual says that it supports microSDHC cards up to 16GB, but people have been using 32GB microSDHC cards without difficulty.
But the 64GB microSDXC cards ("XC" not "HC") are formatted differently and thus, do not work in the Sony PCM-M10, unless you ....
... reformat them to FAT32!
This evening, I received one of these in the mail, from Amazon:

I then downloaded the Windows version of a freeware FAT32 formatting utility called GuiFormat.
Here's the author's home page for this freeware.
The Amazon purchase at the link I provided above includes a USB card reader, but I didn't have to use it, because I found that my Toshiba Windows 7 laptop could read and write to the SanDisk 64GB microSDXC card with its default formatting.
I just popped the card into the included full-size SD adapter, and then loaded it into my laptop's card reader, and having downloaded the GuiFormat.exe file, I executed it, and pointed it at the drive letter that Windows had assigned to the SDXC card, then told it to Quick Format the card (in FAT32 format).
That's it - I was then able to load up the card with nearly 64GB of WAV files (a transfer time of 1 hour, from hard drive to SD card).
WARNING: If you use Windows Explorer to examine the SD card before formatting it, you will have to use Task Manager to kill the process for Explorer.exe, before continuing with the format. So, it's best not to use any other application to read the SD card prior to using the GuiFormat utility. If you do, the format utility will display an error that reads: "Failed to open device..." But, again, the solution is to kill Explorer.exe with Task Manager, then proceed with formatting the card.
Here are some screen shots (I chose the volume label name shown, and used these settings):

The Quick Format to FAT32 takes less than 5 seconds to perform.

Windows Explorer Properties shows the empty FAT32 card, ready for use.

Having transferred files from the hard drive, it's ready for a test drive in the Sony PCM-M10.
And it works flawlessly!
The PCM-M10 doesn't create playlists or maintain any kind of database or catalog. All it has to do is scroll through the root folder or through artist-album folders. So, it seems the sky is the limit, as long as the card is formatted FAT32 and you don't exceed some unknown maximum number of files in any given folder.
I'm really happy to get 64GB cards working with the Sony PCM-M10 as that cuts the card-swapping frequency in half.
Joy! 
Mike
Edited by zilch0md - 2/19/13 at 6:13pm