System 2.1

System 2.1 was released in September 1985 and introduced the HFS (Hierarchical File System) which introduced a real directory structure at the file system level. This version was created specifically to support the Hard Disk 20 and only implemented HFS in RAM, not on the system ROM; startup and most floppy disks remained MFS 400 K volumes. 
This meant that to boot a Macintosh 512k computer off a Hard Disk 20 drive, you had to first insert a System 2.1 boot floppy which would load HFS into RAM, and then switch to the HFS-formatted Hard Disk 20 to load it's copy of System 2.1, ejecting the original boot floppy. 
On the Macintosh 512ke and Plus, HFS was loaded from ROM and the boot floppy was not needed.  System 2.1 would not load HFS on the Hard Disk 20 on the Macintosh 128k, as it did not have enough memory to support HFS.
==External links==
*Classic Mac_OS: System 1, 2, 3, and 4 at Wikipedia
*History of Mac OS at Wikipedia