Add to your file system an efficient sub-system for managing
free blocks, and the ability to have nested sub-directories.

On formatting, a list of unused blocks must be created and
stored safely on the disc in some way. WHen a file is created,
the blocks it needs must be taken from that list in an
efficient way, and when your system stops you must make sure
that the updated list is kept safe.

WHen a file is deleted its blocks must be returned to that
list also in a safe and efficient way.

Efficiency has at least three aspects: not overburdening the
disc system with too many unnecessary reads and writes. and
not taking a long time to find a free block or to release
a previously used one. and not needing large amounts of data
in memory.

The details for subdirectories are totally up to you with the
caveat that they must practically usable.