UDF. It was made for optical media, but works just as well on hard drives. About the only caveat I've found is that you have to be pretty particular regarding w

UDF. It was made for optical media, but works just as well on hard drives. About the only caveat I've found is that you have to be pretty particular regarding what format you choose, but I've found that Windows' format for UDF tends to make the most compatible disks.