SD Audio files are copy protected and can only be checked in and out to the SD card using the Panasonic card reader and Jukebox software. A maximum of 3 copies of a file can be checked out at any time and and they can only be checked back in on the computer from which they originated. Copies and backups done by any other method will not be recognised by the KN. For this reason when backing up and restoring the SD card it is better to use the Technics Song Manager software.
Having said that, I thoroughly agree with Joan and Fran that for manipulation of KN song files SD Explorer is an excellent tool and well worth the asking price.
It should also be noted that there have been occasional examples of SD cards of lesser known brands on which the SD Audio software will not work. These cards have been labelled as SD, have the correct number of contacts and appear to work with all other applications.

Cheers
Len C.