what is the max.drive you can use.....

I have a humax fox (500gb drive) humax pvr (fox sat hdr I think) and I have a spare 2tb drive that I could use as an external drive to copy humax files too eg movies and tv programmes etc, (to use as amedia player) is there a maximum drive sizei can use to format to my humax or is there a limit as I would like to copy mor humax files to drive yours bbuk