Firstly, do you have a single partition over 256gb? if so you may have run into corruption. To use a single partition over 256gb you need to format it in 32k clusters using xbpartitioner.
Secondly, are you complying with FATX limitations? The Xbox uses a file system called FATX which is different to that of your P.C and has certain limitations... The filename may not be longer then 42 chars and " ' * $#@!?>< must not be in your filename.
Hope this helps.
