Without third-party patches, Apple DOS can only read floppy disks running in a 5.25-inch Disk II drive and cannot access any other media, such as hard disk drives, virtual RAM drives, or 3.5-inch floppy disk drives.
ProDOS retains the 16-sector low-level format of DOS 3.3 for 5.25 inch disks, but introduces a new high-level format that is suitable for devices of up to 32 MB; this makes it suitable for hard disks from that era and 3.5-inch floppies.