Unit Allocation Size For a very Large PST File

I need to transfer a large (about 50GB) PST file onto my external HD and I would like to know what would be the best format/unit allocation size combo to set my external HD at. Any recommendations?

Setting up a 1TB hard drive to keep my photos and catalog in. What is the best allocation unit size I should assign when I format the HD?
Plex Lucky Patcher