It's been a while, but aren't you able to format floppies as DOS format, not hard drives?
I don't recall ever seeing the option to format a hard drive as DOS format.
Mind you, I can't ever think I've needed to, so I haven't looked that closely.
Even so, FAT has a size limit of 4GB (you can't have a FAT volume larger than this), so even if the OS supports formatting DOS volumes, it might be smart enough to realize that the volume you're formatting is too large for the FAT file system.