Problems with playback freezing when using USB stick on my CDJ-2000

When i am using my usb stick in my cdj 2000's im having problems like freezing and taking long to load up tunes. I have the latest firmware installed and i use the latest version of rekordbox. Sometimes i will have to turn the units off and on again to get them to play again. Can you please tell me if there is any way to fix this :-/