Thanks BradMZ,
Appreciate your help.
I finally got to looking at this again,
Yes. It was happening with all of the files (bar one).
I didn't previously try another USB drive because the one I was using was working fine for the audio files, both transferring to and from the PX560, and also working fine for MIDI files, similarly.
So, ....., I tried another USB. Formatted it in the Casio, followed the same procedure, and, hay presto! Worked fine for all of the files.
This got me thinking, so I went back to the previous USB stick, and tried it in the Casio, but again only one file could be seen on the stick even though the audio and midi were working fine. So I saved the entire \MUSICDAT to my computer, re-formatted the USB stick in the Casio, and copied back the contents to the USB and inserted it in the Casio, and then the Casio could see all the files.....
So I'm thinking, perhaps I did not format this USB in the Casio to start with and that's why it was having problems. Strange though. I've never seen that behaviour on computers -usually the file system works or it doesn't, not some applications having problems and others not. Anyway, all fixed now. THX for the help..