There can be two reasons for this:
1. With USB devices that have just been formatted in FAT32/FAT, quite some time may be required for loading.
2. Quite some time is required for loading when there are many files in the USB device's "PIONEER DJM/DJM350 REC" folder.
Also, quite some time is required for loading when files other than WAV files are included. Do not include files other than WAV files.