Of most recent, whenever I copied to the external USB it would update media than you go to music/folders/ and it would show [empty] despite there being over 1/2 full mp3s. I tried deleting and re-adding folders to the microSD and same problem. Then I put it into another PC and copied some more folders and it worked. Displayed again, back to the old PC - [empty]. So I assumed there might be a problem with the old PC not writing via usb properly or something. Today I tried it in the new PC and it wouldn’t work either, it did the same thing. The files on the card read absolutely ok when you connect it to the usb or an external adapter card but the clip zip does not recognise any media on there.
Since then I’ve re-formatted the card (FAT32), re-copied 1/4 of the media back, checked and still [empty] card. I dug out the old Sansa clip+ and put it into that one and it loads the files instantly. To my understanding, I might have a faulty player?
At this point, and especially since it seems to work OK in your Clip+ I would call SanDisk Tech Support. If they can’t figure it out and/or deem it defective they can arrange for a warranty replacement for you.