A couple of things:

  • If a machine with File Sharing on is running Mac OS 8.1 or higher, even if the client machine isn't, the client can access and manipulate the files on the 8.1 machine perfectly.
  • If you try to access a hard drive on your own machine that is formatted as HFS+ and you're not running 8.1 or higher, all you see is a text file called "Where_have_all_my_files_gone?" that explains, well, where all your files have gone, and how to access them.
  • HFS, otherwise known as Mac OS Standard, divides the disk into a maximum of 65,000 allocation blocks. HFS+ (also called Mac OS Extended) increases this maximum limit to 4.29 billion allocation blocks.