Hi Douglas,
You are not the first one, believe me. There are several members who tried including me amongst them.
It is not possible to replace the harddrive with a larger one.
First of all, the size of the drive has to be set in the servicemenu of the HDR. The HDR will not recognize it´s size automatically. And for instance, the settings doesn´t allow you to store sizes bigger then three digits (999)MB.
But second, the format isn´t just an ordinairy Fat32, NTFS etc. So just inserting an empty formatted disk will not work.
So, you´re probably stuck with just 250GB´s.
There are lots of alternatives like several Settop Boxes with internal drives or mediaplayers with internal disks and NAS function.