Official comment
If the problem is the USB drive in the host unit, it could be:
- a problem with the USB drive
- a problem with the data on the drive (hence the recommendation to format)
- a conflict between the CDJ's USB host and the USB drive's memory manager
- an issue with the CDJ itself
If it's on a linked player, it could also be:
- the network jack on either unit
- the network port has a bent pin
- the ethernet cable between players is faulty
If you've tried using different make/model USB drives and still have this issue, I'd recommend you contact a Pioneer DJ authorized service centre in your region to have your CDJ inspected and/or repaired.