Hi Rog
I had a similar problem some time ago, I don't know if its related to your problem.
I used an SD card to store some audio backing tracks (I'm think using Jukebox) so I could play them through my KN7000. If I loaded two or three CD's worth it worked fine, but if I loaded a couple more CD's onto the same SD card, my KN7000 would show the error message "The SD card appears to be empty" (or words to that effect).
I contacted Panasonic who suggested I upgrade the KN7000 to version 1.6, they sent me the disc's and that fixed the problem. I now have seven cd's worth of backing tracks on a SD card and now fully recognised by my KN.
Glen