Core: fixs the chunk>>=1 problem

Core: fixs the chunk>>=1 problem

well, it seems ugly, but it can do the right work(I think). here is the new way:
    chunk = chunk == 1 ? 0 : ((chunk+1) >> 1);
if we are reading ONE sector, and after six retries, we can mark it as _failed_.
So just make it be ZERO.