Saving Files In FRDM-K64F

Dear all,
My platform is “FRDM-K64F”and I am having trouble with downloading files to my platform. Here is the problem. Although the Status LED flickers every time I try to save a file to the desk, all of sudden, it always automatically eject and will be reconnected itself in a few seconds. And there is no any files downloaded, but with a fail.txt which always says “TIMEOUT”. This “eject & reconnected” thing not only happens while saving files, but also happen sometimes even I haven’t done anything.

I would be very grateful if someone can help me with this.