Plain hashes are vulnerable to all sorts of attacks. Here's a paper on proof of retrievability https://cseweb.ucsd.edu/~hovav/dist/verstore.pdf and one on non-outsourceabe storage http://cs.umd.edu/~amiller/nonoutsourceable.pdf
Another one is to pretend that you have lots of chunks, and when someone challenges you to produce one, you quickly download it from another source and present it as if you had it all along. That's what the second paper prevents.