I have formatted my 100MB disk and already 6MB is used. Why ?

When you format an PFS-2 disk an area is "reserved" for directory and bitmap information. This allows very fast access to this information. Hence the quick directory access.

Note PFS-2 typically stores MORE data on a disk than FFS. FFS used a whole block for each file in a directory. It also uses "extension" blocks for larger files. PFS-2 does this with a few bytes (Depends on name length).