as you usually read full sectors when accessing the hdd, you should use normal file i/o for it. allocate a buffer that is big enough to keep the maximum amount of data the hex editor can display and read it from disk just the way you to with a normal file. memory mapping 64gb can only work with 64-bit os, as it requires a very big virtual address space. but i haven't tried to map that much data so far.
Don't try it, just do it! ;-)