Mac OS 8.1: What is Mac OS Extended (HFS )? Mac OS 8.1: What is Mac OS Extended (HFS )? by CNET staff @CNET January 23, 1998 12:00 AM PST Updated: September 2, 2009 5:36 PM PDT comments 0 facebook twitter linkedin googleplus more more + email tumblr stumble delicious reddit pinterest digg presented by For those of you new to this matter, FWB has provided a useful summary of exactly what HFS is. Meanwhile, Eric Belsley of the MRP continues his analysis of the inner workings of HFS . This information is also included in our joint Mac OS 8.1 Special Report.