a backup should only take a minute. If the LEDs still flash there might be an issue with the stick. Please try to reformat it to FAT32 in your computer and try again or use a different stick. Make sure that the stick is no bigger than 64 GB.
Got the same issue, I took the same USB stick I used with my toaster without any issues. ( I did format it to FAT32) It doesn`t work on the player