I cannot find this problem anywhere on the 'net, which usually means I'm doing something wrong! 
- DF Player: MP3-TF-16P Version 3. I have three of these.
- TF Cards: 3x cheap knockoffs, 2x SanDisk (total 5 cards, all formatted FAT32) Knockoffs brand new, SanDisk's have been in the drawer for a few years but still fine.
- Connecting via Arduino Uno at the moment, using circuit below (connection/communication is not an issue)
- I understand about the required file structure, voltage divider, etc.
The problem: out of the five TF cards I have, only one of them is recognized by any of the three the DF Players I have. The problem is not the DF modules, I can take the "good" card with mpg/0001.wav ( or 0001.mpg, tried both) and put it in the other two modules, plays fine.
I put the "bad" cards in other devices (my Linux box and wife's Windows box), they mount fine, I can create folders, move files etc.
I've reformatted a couple of them to FAT32, one with long erase. Still aren't recognized.
Tried several disk tools, as mentioned reformatted, nothing is showing any bad sectors or anything wring with these cards. No errors copying to/from. By all indications, they are the same as the "good" card.
I can literally connect the Arduino/DF player up, remove the card while it's powered, put the "good" card in, sound starts playing. Remove the "good" card, try any of the other 4 cards, nada.
Probably not helpful, but when the "good" card is mounted, the LED on the module goes red when playing. When the "bad" cards are in, no light activity at all, which (I think) tells me the module doesn't know it has a card inserted.
These little players appear to have a poor reputation but they do exactly what I need if I can get them all to work, does anyone have any ideas what I could try? Next up is throwing tons of cards at it until I get two more to work LOL
For reference, this is the circuit I'm using with one exception, the VCC for the DF boards is not powered by the breadboard power supply, not the Arduino 5v (swapped that back too, didn't work.)
EDIT: I'm aware of the sensitive file structure and a comment in an article about hidden files got me thinking . . . there was a .Trash directory on the disk, but removing it didn't fixe it:
$ ls -lah /media/bill/'4 GB Drive'/mpg
total 260K
drwxr-xr-x 2 bill bill 4.0K Feb 17 12:42 .
drwxr-xr-x 3 bill bill 4.0K Dec 31 1969 ..
-rw-r--r-- 1 bill bill 250K Feb 17 12:42 0001.wav