Music Mystery: copied music disappears from MMC over and over...

[WimYogya]

Hello,

The following Music Mystery has probably NOTHING to do with MusicBee - but I have no clue what else cause the trouble. Maybe somesone here has an idea.

After I bought a new music player (FIIO) I bought a new SanDisk 128gb micro-sd MMC for my 200 playlists.
I started copying all the content of the selection made from inside MB to that MMC.

And whatever I try: in the beginning all is ok. But after around 40 playlist folders are filled, the next ones totally disappear. That is: in the beginning all seems ok, but the next day all tracks are gone. Not deleted, because nothing found in the recycle bin.
The affected folders are labeled 'empty' and the properties show 0 bytes.
But...
If I want to copy them AGAIN, from my pc, the message says: not enough space...
Then I formatted the MMC a few times and started all over.
But the whole Music Mystery then starts all over as well: same neverending trouble.

The MMC is brandnew and SanDisk has a good reputation. Could the MMC still be the trouble?
Chkdsk did not find any disk error.

Or could the trouble be the result of errors in my recently installed Windows 10 (upgrade from W7) with problematic file/copy management features??

Suggestions highly appreciated.
Thanks,
Wim