@SMALLPAUL > A little more info would be helpful please.
Are you trying to copy the files back from the SD card into your Rekordbox database?
Does the prompt appear automatically in Rekordbox when you insert the SD card back into your computer?
Do I understand correctly that you've tried the exact same thing with the USB device and it does not report an error?
Have you tried reformatting the SD card yourself with a 3rd party app on Windows or Disk Utility on Mac?