@Paul > If you have a USB device and you format it, it will wipe EVERYTHING from that device.
Pioneer players have ALWAYS required flash media to be formatted to FAT32 - that includes SD cards / USB sticks / external harddrives. If your media isn't formatted to FAT, the players won't be able to read the media from it.
If the USB is displaying as empty then either it is actually empty or it is not formatted correctly.
The XDJ doesn't require you to back up and erase everything. The video was simply making the point that if you had a USB stick that wasn't formatted to FAT and contained data, then you should back up the data, format the USB stick to FAT and then transfer the data back to the stick.